- Email: [email protected]
Optimising the cam profile of an electronic unit pump for a heavy-duty diesel engine
Energy 83 (2015) 276e283
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Optimising the cam profile of an electronic unit pump for a heavy-duty diesel engine Tao Qiu a, *, Hefei Dai a, Yan Lei a, Chunlei Cao b, Xuchu Li a a b
College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, China School of Mathematics and Statistics, Beijing Institute of Technology, Beijing, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 July 2014 Received in revised form 15 January 2015 Accepted 10 February 2015 Available online 10 March 2015
For a fuel system with a tangent cam or a constant-velocity cam, the peak injection pressure continues to rise as the injection duration increases, but overly high peak pressures induce mechanical loads and wear, limiting the maximum engine speed and injection quantity. To improve the performance of an EUP (Electronic Unit Pump) fuel system for heavy-duty diesel engines, this work proposes a new pump cam, namely the constant-pressure cam. It helps the EUP run at a higher speed and deliver larger fuel quantities while maintaining a constant peak injection pressure, which improves the power of the heavy-duty diesel engine. A model based on the EUP was built to determine the three constraints for optimising the constant-pressure cam: 1) the pump pressure should equal the nozzle pressure; 2) the cam speed should decrease with the increase in the injection duration; and 3) the cam acceleration gradient should be zero. An EUP system was tested with the tangent cam and the optimised cam under different conditions. The experimental results show that the EUP system with the optimised cam delivers more injection quantity and runs at higher engine speeds while maintaining the same peak pressure as the tangent cam. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Diesel engine EUP (electronic unit pump) Fuel injection Cam profile Optimal design
1. Introduction The high-pressure fuel system greatly influences fuel injection, fuel spray, combustion, and diesel engine output power and emissions under both steady-state and transient conditions [1e3]. The electronically controlled high-pressure fuel systems, such as the CRDI (common rail direction injection) fuel system, the EUI (electronic unit injector) fuel system, the EUP (electronic unit pump) fuel system, the PPN (pump-pipe-nozzle) system and the inline fuel system, are widely applied in heavy-duty engines, due to its good features such as the very high injection pressure, precise control of fuel quantity and great system efficiency. In modern diesel engines, CRDI, is highly flexible in electronically control strategies in terms of high fuel injection pressures, exact injection timing and multi injections; the fuel pressure in this fuel system has little direct relationship with the engine speed since the fuel is reserved in the rail and always kept at a high level. These advantages of CRDI benefit it to provide the engine high enough injection
* Corresponding author. Tel.: þ86 18618323651. E-mail address: [email protected] (T. Qiu). http://dx.doi.org/10.1016/j.energy.2015.02.021 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
pressure even at low engine speed and high engine load and speeds, which is good to meet the varied demands at different engine conditions and suitable for vehicles and power units [1,4,5]. Nowadays, CRDI has been applied more and more in diesel engines, especially in the area of passenger cars. It is a dominating trend that CRDI system gradually replaces other traditional fuel systems, such as the EUI (electronic unit injector) fuel system, the EUP (electronic unit pump) fuel system, the PPN (pump-pipe-nozzle) system and the in-line fuel system, which utilise a cam to directly drive the fuel pump to build fuel pressure [6e8]. However, it is difficult to design and machine those complicate components in the pump and injector of CRDI system; there are several companies all over the world can produce the CRDI products such as BOSCH, Delphi and Denso [9,10]. On the contrary, in China, some companies are still working on EUP, EUI, and PPN systems, for example WEITE Co., YAXINKE Co., since the components of those systems are easier to be machined. In addition, CRDI system has high requirement on fuel quality, but in some application environments of the diesel engine the quality of diesel fuel is not good enough to satisfy the high standard and demand for CRDI. The poor diesel fuel may result in problems such as the fuel leak of the injector, and worsen emissions [11]. In this case of poor-quality diesel fuel, those fuel
T. Qiu et al. / Energy 83 (2015) 276e283
277
Nomenclature a Al An Ap cp E k L m pb pn pp QB Qn un up v
mn r
sound velocity of the fuel (m/s) area of the fuel line pipe (m2) area of the nozzle (m2) area of the plunger (m2) velocity of the plunger (m/s) elastic modulus of the fuel (Pa) integer number length of the pipe (m) integer number back pressure after the nozzle (Pa) fuel pressure at the end of the nozzle (Pa) fuel pressure at the end of the pump (Pa) fuel delivery mass flow from the pump (kg/s) fuel delivery mass at the nozzle (kg/s) fuel velocity at the end of the nozzle (m/s) fuel velocity at the end of the pump (m/s) volume of the fuel (m3) flow coefficient of the nozzle fuel density (kg/m3)
systems such as EUPs which require not too high quality of the diesel fuel are adopted [12,13]. Hence, those EUI and EUP highpressure fuel systems that build pressure based on the pump cam kinematics are applied in heavy-duty diesel engines for trucks, mining machines, power units, and marine engines since those fuel systems are easy to realise simple electronically control and design, even suitable for poor-quality diesel fuel [14,15]. For the fuel system utilising a cam to direct build high pressure, the pump cam is an important component for boosting fuel pressure for the fuel injection. The cam drives the plunger to compress the fuel and build the fuel pressure. The cam profile has a dominate influence on fuel system injection performance parameters such as the injection rate and injection quantity. The cam profile determines the movement of the plunger and builds up the fuel pressure [16e18]. Therefore, it is important to have an optimised cam profile for the fuel system. Yao et al. reported that the EUP fuel system was a complex nonlinear system and that the characteristic parameters such as the fuel supply pressure and the cam profile had a significant correlation with cycle fuel injection quantity [19]. Many researchers have designed cam profiles based on model analyses. Sundarraman et al. proposed a methodology to model the fuel-injection pump used in a diesel engine and reported that changing cam profiles affects the pump [20]. Akinori Miura et al. simulated a concave cam to gain better injection characteristics of the fuel injection pump [21]. Kegl et al. researched optimisation designs of the cam profile with the goal of controlling the injection rate and fuel spray characteristics [22e24]. Those studies showed an effective way of improving the injection characteristics by using an optimised cam in the fuel system. There are two common cam profiles used in diesel fuel systems. One is the tangent cam, and the other is the constant-velocity cam, as shown in Fig. 1. The tangent cam profile is easily machined and is widely applied to in-line fuel pump systems [21,25]. The cam velocity rapidly increases with the increase in the cam angle, and the fuel pressure continues to rise up to the cam peak. However, its effective operating range only covers a narrow range of cam angles and its performance is markedly affected by the injection advance angle, which significantly limits its application to varied engine operating conditions. The constant-velocity cam is widely used in modern electronically controlled high-pressure fuel systems. The
Fig. 1. Cam characteristics. (a) Plunger velocity, (b) plunger lift, (c) delivery pressure.
cam keeps a constant-velocity during the dwell interval, while the delivery pressure still increases to its peak as the cam angle continues to increase. This constant-velocity cam profile allows the electronically controlled fuel pump to run under varied operating conditions. The delivery pressure curves of these cams are shaped like a triangle, giving them the name “triangle characteristic cam”. Modern heavy-duty diesel engines have been enhanced to produce more power and torque, and it is necessary and important for the electronically controlled fuel system to increase the cycle fuel injection quantity. For the triangle characteristic cam, the delivery fuel pressure continues to rise with the increase in the injection duration as the engine load increase, as shown in Fig. 2. This triangle characteristic may produce high fuel pressure for the fuel injection. For low load engine conditions with short injection durations, it is necessary to provide a sufficiently high pressure for the fuel injection. Therefore, the cam profile is designed to operate at a
278
T. Qiu et al. / Energy 83 (2015) 276e283
2. New constant-pressure cam concept
Fig. 2. Delivery fuel pressure of constant-pressure injection cam for heavy-duty diesel engine.
minimum limit velocity. This triangle characteristic cam profile generates higher pressure for higher load conditions with longer injection durations. However, with the increase in fuel injection pressure to approximately 200 MPa, the mechanical parts of the fuel systems now face tougher challenges. Under high pressure operating conditions, there are serious mechanical issues for the fuel system, such as cam wear, noise and fuel leaks [26e28]. To avoid leaks and mechanical wear, the cam system is designed to control the peak pressure below a threshold pressure. Consequently, the fuel injection quantity and the output torque of the engine are also limited. The triangle characteristic cam system has another issue; the peak pressure often occurs at the end of the injection pulse. With such high pressure, it is difficult for the fuel system to sense the rapid pressure drop and the fuel cut off, which can cause other abnormal injection phenomena and finally deteriorate the fuel conversion efficiency. Mehenny et al. reported that the circumferential waviness of the cam surface influences the lubrication of the automotive cam and its flat faced follower [29]. Nayak et al. investigated the rate of wear of cam followers and built a model to predict the wear [30]. They reported that the wear increased with the rise in contact pressure, and permissible contact pressure values could be achieved with proper cam profile design. Thus, it is important for the cam system to have the proper cam profile. To improve the performance of heavy-duty diesel engines, such as marine engines, military vehicle engines and construction machine engines, which generally operate at high loads, it is important to have sufficient cyclic fuel injection quantity and consistently high injection pressures, without excessive peak pressure. This paper seeks to optimise the design of the cam profile for a highpressure EUP fuel system on a heavy-duty diesel engine. First, a new pump cam profile concept was proposed based on the analysis of the theoretical model. Then, a theoretical model of the EUP was built to analyse the fuel injection characteristics and determine the optimisation constraints of the cam. Next, the newly designed cam, based on the optimisation constraints, was machined and matched with the pump. Finally, the fuel system with the new cam was tested under different operating conditions and the injection performance was analysed.
As discussed above, the continuously increasing fuel pressure approaching the end of the injection cycle followed by the rapid pressure drop has negative effects on the cam wear and fuel injection performance. It benefits the fuel system to have an ideal pressure curve that maintains constant peak pressure to the end of the injection cycle, as shown in Fig. 2. This ideal constant delivery pressure curve consists of three periods. The first period (I) is the primary injection period, when the injection begins and the pressure continues to increase. The second period (II) is the steady injection period, when the peak pressure remains constant. The last period (III) is the end period, when the injection stops and the fuel pressure rapidly drops. The ideal constant-pressure curve has characteristics similar to the triangle pressure curve during period I and period III. However, the constant-pressure curve has a steady interval in period II as the peak pressure remains constant. As a result, attention is focused in this work on the optimisation of the design of the second period. Compared to the triangle characteristic, the shadow area under the constant-pressure curve is adjusted to match the shadow area under the triangle curve, ensuring sufficient injection quantity. In addition, the shadow area under the pressure curve becomes larger as the injection duration increases, increasing the injection quantity as the engine load increases. A modest increase of the injection duration may help the fuel system to provide enough fuel quantity for producing high power, but should avoid too long to prolong combustion deep into the expansion stroke and deteriorate engine's efficiency. This ideal constant-pressure cam improves fuel system performance, particularly for heavy-duty diesel engines. Therefore, with careful design of the cam profile, this ideal pressure curve gains advantages such as high fuel quantity, high delivery fuel pressure, rapid injection cut off, precise fuel injection quantity, and reduced wear and leakage. 3. Theoretical analysis 3.1. Model of the EUP The EUP (electronic unit pump) fuel system consists of three critical parts: the pump, the fuel pipe, and the injector. The pump consists of a cam and a plunger. As the EUP system runs, the cam drives the plunger to compress the fuel to generate pressure. Then, the high-pressure fuel flows to the high-pressure fuel pipe and finally to the injector. The high pressure of the fuel forces the needle to lift and open the nozzle to initiate the injection. According to the operating principle, the EUP fuel system can be described by a simplified one-dimensional (1D) flow model in the pipe, as shown in Fig. 3. The fuel in the pump is compressed to the pressure pp. Then, the compressed fuel flows at the speed up from the pump to the pipe and finally to the nozzle. The fuel in the nozzle with the pressure pn is injected into the cylinder, which has a back pressure pb. 3.2. Constant-pressure constraint In this paper, only the injection process of the second period described in Section 2 is discussed. Based on the 1D model of the EUP system, this paper makes the following simplifying assumptions: 1) The temperature differences for the total fuel system are ignored. The fuel temperature tends to maintain stable as the engine normally runs under a steady operation condition.
T. Qiu et al. / Energy 83 (2015) 276e283
279
moment of state 2kþ2, the pressure is sufficient to open the needle, and the fuel flows into the cylinder immediately at a speed of un,2kþ2. Both the fuel flow of the pump and the nozzle are described by Equation (1).
For the pump :
pp;2kþ1 pn;2k up;2kþ1 un;2k ¼ E a
(1-1)
For the nozzle :
pn;2kþ2 pp;2kþ1 up;2kþ1 un;2kþ2 ¼ E a
(1-2)
Here, k is an integer; E is the elastic modulus of the fuel. To solve Equation (1), it is necessary to add an auxiliary relationship equation. For the pump with the constant-pressure cam, the fuel pressure of the nozzle must remain stable, as shown in Equation (2).
pn;2kþ2 ¼ pn;2kþ4 ¼ pn;2kþ6 ¼ / ¼ pn;2kþð2mþ2Þ
(2)
Here, m is any integer. As the injection begins, the fuel flows at a speed of un,2kþ2 determined by Equation (3). Fig. 3. Model of unit pump fuel injection system.
un;2kþ2 2) The fuel is constant within each combined volume of the fuel system. 3) The elastic deformations of all mechanical parts are ignored. 4) The total system is considered to be a perfect system without leaks or flow losses. 5) The solenoid valve of the pump is completely closed, and the nozzle needle is fully opened. During the transfer of compressed fuel from the pump to the pipe and finally to the nozzle, a pressure wave is induced. At the beginning of fuel compression, the fuel pressure wave moves at a speed of up from the pump to the nozzle, and this process persists for a time of L/a for the pressure wave (L is the length of the pipe, and a is the sound velocity of the fuel). The pressure of the nozzle pn is not sufficient to open the sealed needle, causing the fuel flow velocity un to drop to zero. Then, the pressure wave is reflected from the nozzle back to the pump. The pressure of the nozzle pn is repeatedly boosted by the pressure wave and eventually increases to the opening threshold to begin injection. This pressure transfer process is illustrated in Fig. 4. In this figure, odd numbers represent the pump while even numbers represent the nozzle. At the
Fig. 4. Pressure wave transfer process.
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi u u2 p p t n;2nþ2 b am An ¼ n E Al
(3)
Here, mn is the flow coefficient of the nozzle; An is the area of the nozzle; Al is area of the fuel pipe; pb is the back pressure, i.e. the atmosphere pressure, and it is a constant. In Equation (3), for a given fuel system and fuel, mn, An, Al, pb, and E are all constant. Hence, the relationship between un,2kþ2 and pn,2kþ2 may be induced from Equation (3), described as pn;2kþ2 ¼ f ðu2n Þ. In Equation (1), pn,2kþ2 has a first-order relationship with un,2kþ2, while the relationship between pn,2kþ2 and un,2kþ2 is of the second order in Equation (3). The first-order relationship is represented by a linear curve, while the second order is illustrated by a parabolic curve, as shown in Fig. 5. At the beginning of the injection t0, it takes the pressure wave L/ a seconds to reach the nozzle. At this moment, the needle is still sealed with a speed of un ¼ 0 and a pressure p ¼ pn,2k, which is marked as state 0 in Fig. 5. Then, after L/a seconds the pressure wave is reflected back to the pump and marked as state 1, with a speed of u ¼ up,2kþ1 and a pressure of p ¼ pp,2kþ1. As the pressure
Fig. 5. Pressure against velocity after injection.
280
T. Qiu et al. / Energy 83 (2015) 276e283
returns to the nozzle again (state 2), the needle is fully opened for injection, with a pressure p ¼ pn,2kþ2 > pp,2kþ1. The pressure wave is reflected to the pump again and then to the nozzle. Under the pressure conditions of Equation (2), we can deduce from Equations (1) and (3) that state 2 is a stable state where the pump pressure is in balance with the nozzle pressure, i.e.,
pp ¼ pn ¼ constant
(4)
Equation (4) is the first constraint for the constant-pressure cam.
pressure of each volume in the EUP system will not change over time. Thus, we determine that dQB =dt ¼ 0. From Equation (11), it may be deduced that 00
rAp c0p ¼ vr 2r0 Ap cp
(12)
Meanwhile, the fuel properties are described as the following relationships:
r > 0; r0 ¼
00 dr dp dr $ > 0; > 0; r < 0 dp dt dp
The fuel pump has the following kinematic characteristics:
3.3. Kinematic constraints
Ap > 0; v > 0; cp > 0 With the pressure in the fuel pipe equalised according to the section above, we focus on the fuel compression process in this section. Kinematic constraints are derived based on the EUP model. As the cam rotates, the fuel within the sealed plunger chamber is compressed by the moving plunger. For the time period of dt, the fuel quantity remains constant after compression according to the mass conservation law. The fuel quantity before compression is equal to the quantity after compression, as described in Equation (5).
rv ¼ ðr þ drÞðv þ dvÞ þ QB dt ¼ rv þ rdv þ vdr þ drdv þ QB dt Here, r is the fuel density; v is the volume of the fuel within the plunger chamber before compression; QB is the fuel mass flow. Equation (6) is derived from Equation (5).
(6)
The second-order differential expression drdv can be neglected. Then, Equation (7) is derived.
QB dtz rdv vdr
(7)
Based on Equation (7), we can determine:
QB ¼ r
dv dr v ¼ rv0 vr0 dt dt
(8)
Both sides of Equation (8) are differentiated with respect to time t; then, 00 00 00 00 dQB ¼ r0 v0 rv v0 r0 vr ¼ 2r0 v0 rv vr dt
(9)
For this 1D EUP fuel system, the unit compressed fuel volume dv is related to the size of the plunger, as shown in Equation (10).
dv ¼ Ap cp dt 0
v ¼ Ap cp 00
v ¼ Ap c0p
(10-1) (10-2) (10-3)
Here, Ap is the area of the plunger, and cp is the velocity of the plunger. Equation (11) is deduced by substituting Equation (10) into Equation (9). 00 dQB ¼ 2r0 Ap cp þrAp c0p vr dt
(11)
According to the constraint of constant pump pressure, the fuel delivery quantity is also constant. The fuel flow within the total EUP system is considered to be steady flow, meaning that the fuel
(14)
All of these relationships in Equations (13) and (14) are substituted into Equation (12), and the right side of the equation is less than zero. As a result, the left side of the equation must also be less than zero, as described by Equation (15).
c0p < 0
(15)
Furthermore, Equation (16) is determined by the differentiation of both sides of Equation (12) with respect to time t. 00
(5)
QB dt ¼ rdv vdr drdv
(13)
00
000
00
rAp cp ¼ Ap cp r þ vr 2r Ap cp 2r0 Ap c0p
(16)
000
In the model, r << 0 and is negligible. According to the relationships of 13, 14 and 15, we determine that the right side of Equation (16) is greater than zero. Therefore, the left side is also greater than zero, i.e., 00
cp > 0
(17)
Compared to the first two constraints, this constraint is secondorder and has less influence. Regarding the mechanical manufacture of the cam, constraining the cam acceleration gradient above zero requires a section of concave cam profile, which is not machinable. Therefore, for the purpose of manufacture, this third constraint is adjusted to make the cam acceleration gradient equal to zero, as follows: 00
cp ¼ 0
(18)
Thus, three constraints for the constant-pressure cam are derived, i.e., Equation (4) and the relationships of 15 and 18. These three constraints state that 1) pp ¼ pn ¼ constant, the pump pres0 sure is equal to the nozzle pressure; 2) cp < 0, the cam speed de00 creases as the injection duration increases; and 3) cp ¼ 0, the cam acceleration gradient is zero. 4. Optimising the design of the cam profile All three constraints are analysed based on the fuel system model running at a constant cam rotation speed. Therefore, the cam profile optimisation design constraints are suitable for a stable engine speed. In this work, the engine speed for rated engine power is selected as the objective design speed. To verify the optimisation method of the constant-pressure cam, a pump cam of a heavy-duty diesel engine equipped with an EUP system is analysed and the cam profile is re-designed based on the three constraints discussed above. The rated power of the baseline engine is produced at 2100 rpm, so the objective cam is designed to run at 1050 rpm. The details of the EUP fuel system with the baseline tangent profile cam are listed in Table 1. Fig. 6 shows the cam characteristics of both the baseline engine cam and the newly designed cam. Here, we present the lift and
T. Qiu et al. / Energy 83 (2015) 276e283 Table 1 Specifications of baseline EUP fuel system. Parameter Engine stroke Rated engine speed Max. cam lift Max. injection duration Plunger diameter Peak fuel delivery pressure Nozzle holes number Nozzle hole diameter Pipe length Pipe inner diameter
281
Table 2 Test apparatus. Unit
Value
Type Pressure sensor
Kistler 4067C3000
r/min mm CA mm MPa e mm mm mm
4 strokes 2100 18 20 (cam angle) 12 160 8 0.27 400 1.6
Fuel injection analyser
AKRIBIS II
velocity of the constant-pressure cam profile based on the constraints discussed in Section 3. The constant-pressure cam has a lift curve similar to the tangent profile cam, and the maximum lift is equal to that of the tangent cam. In addition, the constant-pressure cam has the same operating period as the baseline cam (the maximum injection duration is less than 35 CA). However, compared to the baseline tangent cam, this optimised cam has a velocity characteristic that reduces the velocity gradually and evenly after it reaches its peak. The lower deceleration mitigates the impacts between mechanical parts. 5. Experimental results and discussion To verify the design constraints of the cam, the redesigned cam was machined and matched to the EUP fuel system. The total EUP system was tested on a test bench under varying operating conditions. On the test bench, the pump was driven by a motor to regulate the speed of the pump camshaft. There was a timing tooth on the camshaft to achieve the correct injection phase. A Kistler pressure sensor was mounted to detect the pressure just before the injector. An AKEIBIS II common rail fuel injection analyser was used to collect the fuel and directly measure the volume under steady state conditions, as well as the pressure rise in the chamber caused by injection. The data acquisition system collected the high pressure signals, the driving and synchronisation signals, the fuel delivery rate, and the pump rotation speed. All test data were transferred to a computer for post-processing. The specifications of the test apparatus and sensors are described in Table 2. First, the EUP system with the baseline cam was tested. During the test, the cam speed was maintained at 1250 rpm while the injection advance angle and the injection duration were changed. The injection pressure and the injection quantity were measured
Fig. 6. Characteristics of the constant-pressure cam and tangent cam.
Specification Range Sensitivity Endpoint linearity Natural frequency Operating temperature Thermal zero shift Thermal sensitivity shift Average delivery Accuracy Timing measurement
0~3000 bar 3.33 mV/bar <±0.5% F.S.O. >200 kHz 0~120 C <±1% FSO <±1% F.S.O. 1~400 mm3 ±0.5 mm3 5 ms
under each test condition. For each steady operation condition, the test data were recorded at least 10 cycles. For the test data of total 10 cycles, both the peak injection pressure and injection quantity are averaged. The COV (coefficients of cyclic variation) of both the measured peak injection pressure and fuel injection quantity are no more than 5%. Fig. 7 shows the tested injection pressures of the EUP with the baseline cam at 1050 rpm. The pressure curve clearly has a triangle characteristic under different injection quantities. The peak injection pressure rises as the injection quantity increases. The baseline cam produces a maximum injection pressure of 160 MPa with an injection quantity of 250 mm3/cycle. The injection characteristics of the EUP system with the optimised constant-pressure cam were tested to compare the injection performance to the baseline. Fig. 8 shows the test results of the EUP with the new machined cam. The operating cam angle covered by the cam lift varies, meaning the injection duration changes to supply the injector with different injection quantities that range from 95 to 331 mm3/cycle. The injection pressure curve also has a small triangle characteristic when the injection quantity is relatively low, i.e., 95 mm3/cycle, 131 mm3/cycle and 209 mm3/cycle. With the increase in the fuel injection quantity, the injection pressure gradually rises. After the injection quantity rises to 290 mm3/cycle, the pressure reaches 160 MPa, which is the peak injection pressure for the baseline cam system. It is notable that the pressure reaches a little bit higher than 160 MPa while the fuel injection quantity continues to increase to 331 mm3/cycle, which is greater than that of the baseline EUP system at 250 mm3/cycle. This slight increase in the peak pressure is induced by adjusting the
Fig. 7. Injection performance speed ¼ 1050 rpm).
of
the
EUP
with
the
baseline
cam
(cam
282
T. Qiu et al. / Energy 83 (2015) 276e283
Fig. 8. Injection pressure of the EUP with the optimised cam (cam speed ¼ 1250 rpm).
constraint of the positive cam acceleration gradient to zero for the purpose of designing a manufacturable cam profile. In addition, the injection duration of the constant-pressure cam is 35 CA as the fuel system delivers the maximum injection quantity.
Fig. 9 compares the optimised cam to the baseline cam. It clearly shows that the peak injection pressures for both cams have a linear increase with the increase in the injection duration. For shorter injection durations, there is a minor difference between these two cams; while for longer injection durations, the peak pressure of the constant-pressure cam is higher than that of the baseline cam, as shown in Fig. 9(a). Meanwhile, the fuel injection quantity of the optimised cam is larger than that of the baseline cam, as shown in Fig. 9(b). These results demonstrate that the optimised cam functions at constant peak pressure value while the EUP system is able to provide larger injection quantities for greater engine loads. At the maximum injection duration (35 CA), the EUP system with the constant-pressure cam has a 32% increase in the injection quantity compared to the baseline cam system. For low engine loads and smaller injection quantities, the constant-pressure cam is also able to produce a sufficient peak injection pressure of 75 MPa. This high pressure for low injection quantities allows the fuel system to function adequately. The optimised EUP system produces the peak pressure of 160 MPa as the cam runs at 1250 rpm, which corresponds to the rated engine speed of 2500 rpm. This rated engine speed is an increase of 19% compared to the baseline speed of 2100 rpm, meaning the EUP system with a constant-pressure cam may produce higher power while running at higher rated speeds. The experimental results show that the performance of the EUP fuel system has been improved by adopting the optimised constant-pressure cam. 6. Conclusions This paper proposes the design criteria for a new pump cam (i.e., the constant-pressure cam) of an EUP system. The constantpressure cam provides consistently high injection pressures but not excessive peak pressure during injection duration, which improves engine performance by providing sufficient cyclic fuel injection quantity, running at higher engine speeds and larger loads. This design criterion is suitable for such high-pressure fuel systems that build up the fuel pressure by pump cam driving. A theory model of an EUP system was completed to analyse the characteristics of the constant-pressure cam. The design criteria of three constraints of optimal design for the constant-pressure cam based on this EUP model were deduced: 1) the pump pressure is equal to the nozzle pressure, i.e., pp ¼ pn ¼ constant; 2) the cam speed should decrease with the increase in injection duration, i.e., 00 c0p < 0; and 3) the cam acceleration gradient should be zero, cp ¼ 0. To verify the optimal designed criteria, the EUP system was tested under different conditions. The experimental results show that the performance of the EUP system is improved by the optimised cam adopting the proposed design constraints. Compared to the tangent cam system, the EUP fuel system with the new constant-pressure cam produces a similar maximum injection pressure of approximately 160 MPa, with a 32% increase in the maximum fuel injection quantity. Furthermore, the optimised EUP system runs at a 19% higher rated engine speed (2500 rpm) compared to the baseline system (2100 rpm). Acknowledgements
Fig. 9. Comparison between the constant-pressure cam and the baseline cam. (a) Peak injection pressure, (b) injection quantity.
We gratefully acknowledge the financial support for this work from the National Natural Science Foundation of China (51006012), the Open Fund of the State Key Lab of Engines in Tianjing University (SKLE K2012-01), the Inner Mongolia Natural Science Foundation (2014MS0501), The Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (CIT&TCD201304038), and the General program of
T. Qiu et al. / Energy 83 (2015) 276e283
science and technology development project of the Beijing Municipal Education Commission (KM201310005033).
References [1] Magno A, Mancaruso E, Vaglieco BM. Experimental investigation in an optically accessible diesel engine of a fouled piezoelectric injector. Energy 2014;64:842e52. [2] Soid SN, Zainal ZA. Spray and combustion characterization for internal combustion engines using optical measuring techniques e a review. Energy 2011;36:724e41. [3] Rakopoulos CD, Dimaratos AM, Giakoumis EG, Rakopoulos DC. Evaluation of the effect of engine, load and turbocharger parameters on transient emissions of diesel engine. Energy Convers Manage 2009;50(9):2381e93. [4] Agarwal AK, Dhar A, Gupta JG, Kim W, Lee CS, Park S. Effect of fuel injection pressure and injection timing on spray characteristics and particulate sizenumber distribution in a biodiesel fuelled common rail direct injection diesel engine. Appl Energy 2014;130:212e21. [5] Bae J, Kim C. A study on integrated design for improving fatigue life of common rail pipe considering stress concentration at complex shape. J Mech Sci Technol 2014;28(9):3617e27. [6] Joachim SH. New developments in MAN B&W four-stroke marine diesel engines. J Inst Eng (India), Part MR: Mar Eng Div 2002;83:5e11. [7] Chaufour P, Millet G, Hedna M. Advanced modeling of a heavy-truck unitinjector system and its application in the engine design process. SAE paper. 2004-01-0200. 2004. [8] Dierre L. Electronically controlled high pressure unit injection system for diesel engine. SAE paper. SAE 911819. 1991. [9] Huang YQ, Leet J. Investigation of in-cylinder NOx and PM reduction with Delphi E3 flexible unit injectors on a heavy-duty diesel engines. SAE paper. 2008-01-1792. 2008. [10] Catania AE, Dongiovanni C, Mittica A. Implicit numerical model of a highpressure injection system. J Eng Gas Turbines Power 1992;114(3):534e43. [11] Hayat Q, Fan LY, Song EZ, Ma XZ, Tian BQ, Farouk N. Nonlinear modelling and analysis of pressure wave inside CEUP fuel pipeline. Math Probl Eng 2014;2014:11. Article ID 521859. [12] Tian BQ, Fan LY, Hayat Q, Ma XZ, Song EZ, Wang H. Computation of pressure fluctuation frequency in electronic unit pump for diesel engine. J Mech Sci Technol 2014;28(4):1529e37. [13] Wang P, Liu FS, Li XR. Driving characteristics for electronic control unit pump fuel injection system. In: 2012 International conference on computer distributed control and intelligent environmental monitoring. IEEE; 2012. p. 264e7.
283
[14] Xu QK, Zhu KQ, Zhuo B. Simulation in thermal design for electronic control unit of electronic unit pump. Chin J Mech Eng (English Edition) 2008;21(5): 1e7. [15] Yu ST, Yang SW, Yang L. Model-based development of real-time software system for electronic unit pump system. Chin J Mech Eng (English Edition) 2007;20(1):25e30. [16] An XH, Liu BL, Cui T, Xie ZX, Zhang FJ. Electronic control unit development for unit pump diesel engine. In: 2010 IEEE International conference on optoelectronics and image processing, Haikou, China; November 11e12, 2010. p. 480e2. [17] Wang TC, Han JS, Xie XB, Lai MC, Henein NA. Parametric characterization of high pressure diesel fuel injection systems. Trans ASME 2003;125:412e26. [18] Hayat Q, Fan LY, Xiu ZM, Bing QT. Comparative study of pressure wave mathematical models for HP fuel pipeline of CEUP at various operating conditions. Int J Smart Sens Intell Syst 2013;6(3):1077e101. [19] Yao C, Fan LY, Dong Q, Song EZ, Tian BQ. Correlation analysis of factors influencing the electronic unit pump cycle fuel injection quantity under overall operating conditions for diesel engines. J Mech Sci Technol 2012;26(9):2929e39. [20] Sundarraman P, Saha SK, Vasa NJ, Baskaran R, Sunilkumar V, Raghavendra K. Modeling and analysis of a fuel-injection pump used in diesel engines. Int J Automot Technol 2012;13(2):193e203. [21] Miura Akinori, yamaguchi Yuhji. Injection rate control of in-line injection pump e cam design through injection process simulation. SAE paper. SAE 950606. 1995. [22] Kegl B. A procedure for upgrading an electronic control diesel fuel injection system by considering several engine operating regimes simultaneously. J Mech Des 1999;121:159e66. [23] Kegl B. Injection system design optimization by considering fuel spray characteristics. J Mech Des 2004;126:703e10. [24] Kegl B, Kegl M, Pehan S. Optimization of a fuel injection system for diesel and biodiesel. Energy Fuels 2008;22:1046e56. [25] Qiu T, Liu XH, Liu FS, Yu L, Gao Z. Research on the performance of EUP fuel system using tangent cam. Veh Engine 2008;179(6):1e3. [26] Cheng W, Cheng HS, Yasuda Y. Wear and life prediction of cam roller follower. SAE paper. SAE 940822. 1994. p. 468e79. [27] Ivantysynova M, Huang CC, Christiansen SK. Computer aided valve plat design e an effective way to reduce noise. SAE paper. SAE 2004-01-2621. 2004. [28] Lindholm P, Bjorklund S. Cortes MCvo. Characterisation of wear on a cam follower system in a diesel engine. Wear 2003;254:1199e207. [29] Mehenny DS, Taylor CM. Analysis of the influence of cam surface waviness on the lubrication of the automotive came and flat faced follower. Tribol Ser 2001;39:863e72. [30] Nayak N, Lakshminarayanan PA, Gaiendra Babu MK, Dani AD. Predictions of cam follower wear in diesel engines. Wear 2006;260:181e92.
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Optimising the cam profile of an electronic unit pump for a heavy-duty diesel engine Tao Qiu a, *, Hefei Dai a, Yan Lei a, Chunlei Cao b, Xuchu Li a a b
College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, China School of Mathematics and Statistics, Beijing Institute of Technology, Beijing, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 July 2014 Received in revised form 15 January 2015 Accepted 10 February 2015 Available online 10 March 2015
For a fuel system with a tangent cam or a constant-velocity cam, the peak injection pressure continues to rise as the injection duration increases, but overly high peak pressures induce mechanical loads and wear, limiting the maximum engine speed and injection quantity. To improve the performance of an EUP (Electronic Unit Pump) fuel system for heavy-duty diesel engines, this work proposes a new pump cam, namely the constant-pressure cam. It helps the EUP run at a higher speed and deliver larger fuel quantities while maintaining a constant peak injection pressure, which improves the power of the heavy-duty diesel engine. A model based on the EUP was built to determine the three constraints for optimising the constant-pressure cam: 1) the pump pressure should equal the nozzle pressure; 2) the cam speed should decrease with the increase in the injection duration; and 3) the cam acceleration gradient should be zero. An EUP system was tested with the tangent cam and the optimised cam under different conditions. The experimental results show that the EUP system with the optimised cam delivers more injection quantity and runs at higher engine speeds while maintaining the same peak pressure as the tangent cam. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Diesel engine EUP (electronic unit pump) Fuel injection Cam profile Optimal design
1. Introduction The high-pressure fuel system greatly influences fuel injection, fuel spray, combustion, and diesel engine output power and emissions under both steady-state and transient conditions [1e3]. The electronically controlled high-pressure fuel systems, such as the CRDI (common rail direction injection) fuel system, the EUI (electronic unit injector) fuel system, the EUP (electronic unit pump) fuel system, the PPN (pump-pipe-nozzle) system and the inline fuel system, are widely applied in heavy-duty engines, due to its good features such as the very high injection pressure, precise control of fuel quantity and great system efficiency. In modern diesel engines, CRDI, is highly flexible in electronically control strategies in terms of high fuel injection pressures, exact injection timing and multi injections; the fuel pressure in this fuel system has little direct relationship with the engine speed since the fuel is reserved in the rail and always kept at a high level. These advantages of CRDI benefit it to provide the engine high enough injection
* Corresponding author. Tel.: þ86 18618323651. E-mail address: [email protected] (T. Qiu). http://dx.doi.org/10.1016/j.energy.2015.02.021 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
pressure even at low engine speed and high engine load and speeds, which is good to meet the varied demands at different engine conditions and suitable for vehicles and power units [1,4,5]. Nowadays, CRDI has been applied more and more in diesel engines, especially in the area of passenger cars. It is a dominating trend that CRDI system gradually replaces other traditional fuel systems, such as the EUI (electronic unit injector) fuel system, the EUP (electronic unit pump) fuel system, the PPN (pump-pipe-nozzle) system and the in-line fuel system, which utilise a cam to directly drive the fuel pump to build fuel pressure [6e8]. However, it is difficult to design and machine those complicate components in the pump and injector of CRDI system; there are several companies all over the world can produce the CRDI products such as BOSCH, Delphi and Denso [9,10]. On the contrary, in China, some companies are still working on EUP, EUI, and PPN systems, for example WEITE Co., YAXINKE Co., since the components of those systems are easier to be machined. In addition, CRDI system has high requirement on fuel quality, but in some application environments of the diesel engine the quality of diesel fuel is not good enough to satisfy the high standard and demand for CRDI. The poor diesel fuel may result in problems such as the fuel leak of the injector, and worsen emissions [11]. In this case of poor-quality diesel fuel, those fuel
T. Qiu et al. / Energy 83 (2015) 276e283
277
Nomenclature a Al An Ap cp E k L m pb pn pp QB Qn un up v
mn r
sound velocity of the fuel (m/s) area of the fuel line pipe (m2) area of the nozzle (m2) area of the plunger (m2) velocity of the plunger (m/s) elastic modulus of the fuel (Pa) integer number length of the pipe (m) integer number back pressure after the nozzle (Pa) fuel pressure at the end of the nozzle (Pa) fuel pressure at the end of the pump (Pa) fuel delivery mass flow from the pump (kg/s) fuel delivery mass at the nozzle (kg/s) fuel velocity at the end of the nozzle (m/s) fuel velocity at the end of the pump (m/s) volume of the fuel (m3) flow coefficient of the nozzle fuel density (kg/m3)
systems such as EUPs which require not too high quality of the diesel fuel are adopted [12,13]. Hence, those EUI and EUP highpressure fuel systems that build pressure based on the pump cam kinematics are applied in heavy-duty diesel engines for trucks, mining machines, power units, and marine engines since those fuel systems are easy to realise simple electronically control and design, even suitable for poor-quality diesel fuel [14,15]. For the fuel system utilising a cam to direct build high pressure, the pump cam is an important component for boosting fuel pressure for the fuel injection. The cam drives the plunger to compress the fuel and build the fuel pressure. The cam profile has a dominate influence on fuel system injection performance parameters such as the injection rate and injection quantity. The cam profile determines the movement of the plunger and builds up the fuel pressure [16e18]. Therefore, it is important to have an optimised cam profile for the fuel system. Yao et al. reported that the EUP fuel system was a complex nonlinear system and that the characteristic parameters such as the fuel supply pressure and the cam profile had a significant correlation with cycle fuel injection quantity [19]. Many researchers have designed cam profiles based on model analyses. Sundarraman et al. proposed a methodology to model the fuel-injection pump used in a diesel engine and reported that changing cam profiles affects the pump [20]. Akinori Miura et al. simulated a concave cam to gain better injection characteristics of the fuel injection pump [21]. Kegl et al. researched optimisation designs of the cam profile with the goal of controlling the injection rate and fuel spray characteristics [22e24]. Those studies showed an effective way of improving the injection characteristics by using an optimised cam in the fuel system. There are two common cam profiles used in diesel fuel systems. One is the tangent cam, and the other is the constant-velocity cam, as shown in Fig. 1. The tangent cam profile is easily machined and is widely applied to in-line fuel pump systems [21,25]. The cam velocity rapidly increases with the increase in the cam angle, and the fuel pressure continues to rise up to the cam peak. However, its effective operating range only covers a narrow range of cam angles and its performance is markedly affected by the injection advance angle, which significantly limits its application to varied engine operating conditions. The constant-velocity cam is widely used in modern electronically controlled high-pressure fuel systems. The
Fig. 1. Cam characteristics. (a) Plunger velocity, (b) plunger lift, (c) delivery pressure.
cam keeps a constant-velocity during the dwell interval, while the delivery pressure still increases to its peak as the cam angle continues to increase. This constant-velocity cam profile allows the electronically controlled fuel pump to run under varied operating conditions. The delivery pressure curves of these cams are shaped like a triangle, giving them the name “triangle characteristic cam”. Modern heavy-duty diesel engines have been enhanced to produce more power and torque, and it is necessary and important for the electronically controlled fuel system to increase the cycle fuel injection quantity. For the triangle characteristic cam, the delivery fuel pressure continues to rise with the increase in the injection duration as the engine load increase, as shown in Fig. 2. This triangle characteristic may produce high fuel pressure for the fuel injection. For low load engine conditions with short injection durations, it is necessary to provide a sufficiently high pressure for the fuel injection. Therefore, the cam profile is designed to operate at a
278
T. Qiu et al. / Energy 83 (2015) 276e283
2. New constant-pressure cam concept
Fig. 2. Delivery fuel pressure of constant-pressure injection cam for heavy-duty diesel engine.
minimum limit velocity. This triangle characteristic cam profile generates higher pressure for higher load conditions with longer injection durations. However, with the increase in fuel injection pressure to approximately 200 MPa, the mechanical parts of the fuel systems now face tougher challenges. Under high pressure operating conditions, there are serious mechanical issues for the fuel system, such as cam wear, noise and fuel leaks [26e28]. To avoid leaks and mechanical wear, the cam system is designed to control the peak pressure below a threshold pressure. Consequently, the fuel injection quantity and the output torque of the engine are also limited. The triangle characteristic cam system has another issue; the peak pressure often occurs at the end of the injection pulse. With such high pressure, it is difficult for the fuel system to sense the rapid pressure drop and the fuel cut off, which can cause other abnormal injection phenomena and finally deteriorate the fuel conversion efficiency. Mehenny et al. reported that the circumferential waviness of the cam surface influences the lubrication of the automotive cam and its flat faced follower [29]. Nayak et al. investigated the rate of wear of cam followers and built a model to predict the wear [30]. They reported that the wear increased with the rise in contact pressure, and permissible contact pressure values could be achieved with proper cam profile design. Thus, it is important for the cam system to have the proper cam profile. To improve the performance of heavy-duty diesel engines, such as marine engines, military vehicle engines and construction machine engines, which generally operate at high loads, it is important to have sufficient cyclic fuel injection quantity and consistently high injection pressures, without excessive peak pressure. This paper seeks to optimise the design of the cam profile for a highpressure EUP fuel system on a heavy-duty diesel engine. First, a new pump cam profile concept was proposed based on the analysis of the theoretical model. Then, a theoretical model of the EUP was built to analyse the fuel injection characteristics and determine the optimisation constraints of the cam. Next, the newly designed cam, based on the optimisation constraints, was machined and matched with the pump. Finally, the fuel system with the new cam was tested under different operating conditions and the injection performance was analysed.
As discussed above, the continuously increasing fuel pressure approaching the end of the injection cycle followed by the rapid pressure drop has negative effects on the cam wear and fuel injection performance. It benefits the fuel system to have an ideal pressure curve that maintains constant peak pressure to the end of the injection cycle, as shown in Fig. 2. This ideal constant delivery pressure curve consists of three periods. The first period (I) is the primary injection period, when the injection begins and the pressure continues to increase. The second period (II) is the steady injection period, when the peak pressure remains constant. The last period (III) is the end period, when the injection stops and the fuel pressure rapidly drops. The ideal constant-pressure curve has characteristics similar to the triangle pressure curve during period I and period III. However, the constant-pressure curve has a steady interval in period II as the peak pressure remains constant. As a result, attention is focused in this work on the optimisation of the design of the second period. Compared to the triangle characteristic, the shadow area under the constant-pressure curve is adjusted to match the shadow area under the triangle curve, ensuring sufficient injection quantity. In addition, the shadow area under the pressure curve becomes larger as the injection duration increases, increasing the injection quantity as the engine load increases. A modest increase of the injection duration may help the fuel system to provide enough fuel quantity for producing high power, but should avoid too long to prolong combustion deep into the expansion stroke and deteriorate engine's efficiency. This ideal constant-pressure cam improves fuel system performance, particularly for heavy-duty diesel engines. Therefore, with careful design of the cam profile, this ideal pressure curve gains advantages such as high fuel quantity, high delivery fuel pressure, rapid injection cut off, precise fuel injection quantity, and reduced wear and leakage. 3. Theoretical analysis 3.1. Model of the EUP The EUP (electronic unit pump) fuel system consists of three critical parts: the pump, the fuel pipe, and the injector. The pump consists of a cam and a plunger. As the EUP system runs, the cam drives the plunger to compress the fuel to generate pressure. Then, the high-pressure fuel flows to the high-pressure fuel pipe and finally to the injector. The high pressure of the fuel forces the needle to lift and open the nozzle to initiate the injection. According to the operating principle, the EUP fuel system can be described by a simplified one-dimensional (1D) flow model in the pipe, as shown in Fig. 3. The fuel in the pump is compressed to the pressure pp. Then, the compressed fuel flows at the speed up from the pump to the pipe and finally to the nozzle. The fuel in the nozzle with the pressure pn is injected into the cylinder, which has a back pressure pb. 3.2. Constant-pressure constraint In this paper, only the injection process of the second period described in Section 2 is discussed. Based on the 1D model of the EUP system, this paper makes the following simplifying assumptions: 1) The temperature differences for the total fuel system are ignored. The fuel temperature tends to maintain stable as the engine normally runs under a steady operation condition.
T. Qiu et al. / Energy 83 (2015) 276e283
279
moment of state 2kþ2, the pressure is sufficient to open the needle, and the fuel flows into the cylinder immediately at a speed of un,2kþ2. Both the fuel flow of the pump and the nozzle are described by Equation (1).
For the pump :
pp;2kþ1 pn;2k up;2kþ1 un;2k ¼ E a
(1-1)
For the nozzle :
pn;2kþ2 pp;2kþ1 up;2kþ1 un;2kþ2 ¼ E a
(1-2)
Here, k is an integer; E is the elastic modulus of the fuel. To solve Equation (1), it is necessary to add an auxiliary relationship equation. For the pump with the constant-pressure cam, the fuel pressure of the nozzle must remain stable, as shown in Equation (2).
pn;2kþ2 ¼ pn;2kþ4 ¼ pn;2kþ6 ¼ / ¼ pn;2kþð2mþ2Þ
(2)
Here, m is any integer. As the injection begins, the fuel flows at a speed of un,2kþ2 determined by Equation (3). Fig. 3. Model of unit pump fuel injection system.
un;2kþ2 2) The fuel is constant within each combined volume of the fuel system. 3) The elastic deformations of all mechanical parts are ignored. 4) The total system is considered to be a perfect system without leaks or flow losses. 5) The solenoid valve of the pump is completely closed, and the nozzle needle is fully opened. During the transfer of compressed fuel from the pump to the pipe and finally to the nozzle, a pressure wave is induced. At the beginning of fuel compression, the fuel pressure wave moves at a speed of up from the pump to the nozzle, and this process persists for a time of L/a for the pressure wave (L is the length of the pipe, and a is the sound velocity of the fuel). The pressure of the nozzle pn is not sufficient to open the sealed needle, causing the fuel flow velocity un to drop to zero. Then, the pressure wave is reflected from the nozzle back to the pump. The pressure of the nozzle pn is repeatedly boosted by the pressure wave and eventually increases to the opening threshold to begin injection. This pressure transfer process is illustrated in Fig. 4. In this figure, odd numbers represent the pump while even numbers represent the nozzle. At the
Fig. 4. Pressure wave transfer process.
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi u u2 p p t n;2nþ2 b am An ¼ n E Al
(3)
Here, mn is the flow coefficient of the nozzle; An is the area of the nozzle; Al is area of the fuel pipe; pb is the back pressure, i.e. the atmosphere pressure, and it is a constant. In Equation (3), for a given fuel system and fuel, mn, An, Al, pb, and E are all constant. Hence, the relationship between un,2kþ2 and pn,2kþ2 may be induced from Equation (3), described as pn;2kþ2 ¼ f ðu2n Þ. In Equation (1), pn,2kþ2 has a first-order relationship with un,2kþ2, while the relationship between pn,2kþ2 and un,2kþ2 is of the second order in Equation (3). The first-order relationship is represented by a linear curve, while the second order is illustrated by a parabolic curve, as shown in Fig. 5. At the beginning of the injection t0, it takes the pressure wave L/ a seconds to reach the nozzle. At this moment, the needle is still sealed with a speed of un ¼ 0 and a pressure p ¼ pn,2k, which is marked as state 0 in Fig. 5. Then, after L/a seconds the pressure wave is reflected back to the pump and marked as state 1, with a speed of u ¼ up,2kþ1 and a pressure of p ¼ pp,2kþ1. As the pressure
Fig. 5. Pressure against velocity after injection.
280
T. Qiu et al. / Energy 83 (2015) 276e283
returns to the nozzle again (state 2), the needle is fully opened for injection, with a pressure p ¼ pn,2kþ2 > pp,2kþ1. The pressure wave is reflected to the pump again and then to the nozzle. Under the pressure conditions of Equation (2), we can deduce from Equations (1) and (3) that state 2 is a stable state where the pump pressure is in balance with the nozzle pressure, i.e.,
pp ¼ pn ¼ constant
(4)
Equation (4) is the first constraint for the constant-pressure cam.
pressure of each volume in the EUP system will not change over time. Thus, we determine that dQB =dt ¼ 0. From Equation (11), it may be deduced that 00
rAp c0p ¼ vr 2r0 Ap cp
(12)
Meanwhile, the fuel properties are described as the following relationships:
r > 0; r0 ¼
00 dr dp dr $ > 0; > 0; r < 0 dp dt dp
The fuel pump has the following kinematic characteristics:
3.3. Kinematic constraints
Ap > 0; v > 0; cp > 0 With the pressure in the fuel pipe equalised according to the section above, we focus on the fuel compression process in this section. Kinematic constraints are derived based on the EUP model. As the cam rotates, the fuel within the sealed plunger chamber is compressed by the moving plunger. For the time period of dt, the fuel quantity remains constant after compression according to the mass conservation law. The fuel quantity before compression is equal to the quantity after compression, as described in Equation (5).
rv ¼ ðr þ drÞðv þ dvÞ þ QB dt ¼ rv þ rdv þ vdr þ drdv þ QB dt Here, r is the fuel density; v is the volume of the fuel within the plunger chamber before compression; QB is the fuel mass flow. Equation (6) is derived from Equation (5).
(6)
The second-order differential expression drdv can be neglected. Then, Equation (7) is derived.
QB dtz rdv vdr
(7)
Based on Equation (7), we can determine:
QB ¼ r
dv dr v ¼ rv0 vr0 dt dt
(8)
Both sides of Equation (8) are differentiated with respect to time t; then, 00 00 00 00 dQB ¼ r0 v0 rv v0 r0 vr ¼ 2r0 v0 rv vr dt
(9)
For this 1D EUP fuel system, the unit compressed fuel volume dv is related to the size of the plunger, as shown in Equation (10).
dv ¼ Ap cp dt 0
v ¼ Ap cp 00
v ¼ Ap c0p
(10-1) (10-2) (10-3)
Here, Ap is the area of the plunger, and cp is the velocity of the plunger. Equation (11) is deduced by substituting Equation (10) into Equation (9). 00 dQB ¼ 2r0 Ap cp þrAp c0p vr dt
(11)
According to the constraint of constant pump pressure, the fuel delivery quantity is also constant. The fuel flow within the total EUP system is considered to be steady flow, meaning that the fuel
(14)
All of these relationships in Equations (13) and (14) are substituted into Equation (12), and the right side of the equation is less than zero. As a result, the left side of the equation must also be less than zero, as described by Equation (15).
c0p < 0
(15)
Furthermore, Equation (16) is determined by the differentiation of both sides of Equation (12) with respect to time t. 00
(5)
QB dt ¼ rdv vdr drdv
(13)
00
000
00
rAp cp ¼ Ap cp r þ vr 2r Ap cp 2r0 Ap c0p
(16)
000
In the model, r << 0 and is negligible. According to the relationships of 13, 14 and 15, we determine that the right side of Equation (16) is greater than zero. Therefore, the left side is also greater than zero, i.e., 00
cp > 0
(17)
Compared to the first two constraints, this constraint is secondorder and has less influence. Regarding the mechanical manufacture of the cam, constraining the cam acceleration gradient above zero requires a section of concave cam profile, which is not machinable. Therefore, for the purpose of manufacture, this third constraint is adjusted to make the cam acceleration gradient equal to zero, as follows: 00
cp ¼ 0
(18)
Thus, three constraints for the constant-pressure cam are derived, i.e., Equation (4) and the relationships of 15 and 18. These three constraints state that 1) pp ¼ pn ¼ constant, the pump pres0 sure is equal to the nozzle pressure; 2) cp < 0, the cam speed de00 creases as the injection duration increases; and 3) cp ¼ 0, the cam acceleration gradient is zero. 4. Optimising the design of the cam profile All three constraints are analysed based on the fuel system model running at a constant cam rotation speed. Therefore, the cam profile optimisation design constraints are suitable for a stable engine speed. In this work, the engine speed for rated engine power is selected as the objective design speed. To verify the optimisation method of the constant-pressure cam, a pump cam of a heavy-duty diesel engine equipped with an EUP system is analysed and the cam profile is re-designed based on the three constraints discussed above. The rated power of the baseline engine is produced at 2100 rpm, so the objective cam is designed to run at 1050 rpm. The details of the EUP fuel system with the baseline tangent profile cam are listed in Table 1. Fig. 6 shows the cam characteristics of both the baseline engine cam and the newly designed cam. Here, we present the lift and
T. Qiu et al. / Energy 83 (2015) 276e283 Table 1 Specifications of baseline EUP fuel system. Parameter Engine stroke Rated engine speed Max. cam lift Max. injection duration Plunger diameter Peak fuel delivery pressure Nozzle holes number Nozzle hole diameter Pipe length Pipe inner diameter
281
Table 2 Test apparatus. Unit
Value
Type Pressure sensor
Kistler 4067C3000
r/min mm CA mm MPa e mm mm mm
4 strokes 2100 18 20 (cam angle) 12 160 8 0.27 400 1.6
Fuel injection analyser
AKRIBIS II
velocity of the constant-pressure cam profile based on the constraints discussed in Section 3. The constant-pressure cam has a lift curve similar to the tangent profile cam, and the maximum lift is equal to that of the tangent cam. In addition, the constant-pressure cam has the same operating period as the baseline cam (the maximum injection duration is less than 35 CA). However, compared to the baseline tangent cam, this optimised cam has a velocity characteristic that reduces the velocity gradually and evenly after it reaches its peak. The lower deceleration mitigates the impacts between mechanical parts. 5. Experimental results and discussion To verify the design constraints of the cam, the redesigned cam was machined and matched to the EUP fuel system. The total EUP system was tested on a test bench under varying operating conditions. On the test bench, the pump was driven by a motor to regulate the speed of the pump camshaft. There was a timing tooth on the camshaft to achieve the correct injection phase. A Kistler pressure sensor was mounted to detect the pressure just before the injector. An AKEIBIS II common rail fuel injection analyser was used to collect the fuel and directly measure the volume under steady state conditions, as well as the pressure rise in the chamber caused by injection. The data acquisition system collected the high pressure signals, the driving and synchronisation signals, the fuel delivery rate, and the pump rotation speed. All test data were transferred to a computer for post-processing. The specifications of the test apparatus and sensors are described in Table 2. First, the EUP system with the baseline cam was tested. During the test, the cam speed was maintained at 1250 rpm while the injection advance angle and the injection duration were changed. The injection pressure and the injection quantity were measured
Fig. 6. Characteristics of the constant-pressure cam and tangent cam.
Specification Range Sensitivity Endpoint linearity Natural frequency Operating temperature Thermal zero shift Thermal sensitivity shift Average delivery Accuracy Timing measurement
0~3000 bar 3.33 mV/bar <±0.5% F.S.O. >200 kHz 0~120 C <±1% FSO <±1% F.S.O. 1~400 mm3 ±0.5 mm3 5 ms
under each test condition. For each steady operation condition, the test data were recorded at least 10 cycles. For the test data of total 10 cycles, both the peak injection pressure and injection quantity are averaged. The COV (coefficients of cyclic variation) of both the measured peak injection pressure and fuel injection quantity are no more than 5%. Fig. 7 shows the tested injection pressures of the EUP with the baseline cam at 1050 rpm. The pressure curve clearly has a triangle characteristic under different injection quantities. The peak injection pressure rises as the injection quantity increases. The baseline cam produces a maximum injection pressure of 160 MPa with an injection quantity of 250 mm3/cycle. The injection characteristics of the EUP system with the optimised constant-pressure cam were tested to compare the injection performance to the baseline. Fig. 8 shows the test results of the EUP with the new machined cam. The operating cam angle covered by the cam lift varies, meaning the injection duration changes to supply the injector with different injection quantities that range from 95 to 331 mm3/cycle. The injection pressure curve also has a small triangle characteristic when the injection quantity is relatively low, i.e., 95 mm3/cycle, 131 mm3/cycle and 209 mm3/cycle. With the increase in the fuel injection quantity, the injection pressure gradually rises. After the injection quantity rises to 290 mm3/cycle, the pressure reaches 160 MPa, which is the peak injection pressure for the baseline cam system. It is notable that the pressure reaches a little bit higher than 160 MPa while the fuel injection quantity continues to increase to 331 mm3/cycle, which is greater than that of the baseline EUP system at 250 mm3/cycle. This slight increase in the peak pressure is induced by adjusting the
Fig. 7. Injection performance speed ¼ 1050 rpm).
of
the
EUP
with
the
baseline
cam
(cam
282
T. Qiu et al. / Energy 83 (2015) 276e283
Fig. 8. Injection pressure of the EUP with the optimised cam (cam speed ¼ 1250 rpm).
constraint of the positive cam acceleration gradient to zero for the purpose of designing a manufacturable cam profile. In addition, the injection duration of the constant-pressure cam is 35 CA as the fuel system delivers the maximum injection quantity.
Fig. 9 compares the optimised cam to the baseline cam. It clearly shows that the peak injection pressures for both cams have a linear increase with the increase in the injection duration. For shorter injection durations, there is a minor difference between these two cams; while for longer injection durations, the peak pressure of the constant-pressure cam is higher than that of the baseline cam, as shown in Fig. 9(a). Meanwhile, the fuel injection quantity of the optimised cam is larger than that of the baseline cam, as shown in Fig. 9(b). These results demonstrate that the optimised cam functions at constant peak pressure value while the EUP system is able to provide larger injection quantities for greater engine loads. At the maximum injection duration (35 CA), the EUP system with the constant-pressure cam has a 32% increase in the injection quantity compared to the baseline cam system. For low engine loads and smaller injection quantities, the constant-pressure cam is also able to produce a sufficient peak injection pressure of 75 MPa. This high pressure for low injection quantities allows the fuel system to function adequately. The optimised EUP system produces the peak pressure of 160 MPa as the cam runs at 1250 rpm, which corresponds to the rated engine speed of 2500 rpm. This rated engine speed is an increase of 19% compared to the baseline speed of 2100 rpm, meaning the EUP system with a constant-pressure cam may produce higher power while running at higher rated speeds. The experimental results show that the performance of the EUP fuel system has been improved by adopting the optimised constant-pressure cam. 6. Conclusions This paper proposes the design criteria for a new pump cam (i.e., the constant-pressure cam) of an EUP system. The constantpressure cam provides consistently high injection pressures but not excessive peak pressure during injection duration, which improves engine performance by providing sufficient cyclic fuel injection quantity, running at higher engine speeds and larger loads. This design criterion is suitable for such high-pressure fuel systems that build up the fuel pressure by pump cam driving. A theory model of an EUP system was completed to analyse the characteristics of the constant-pressure cam. The design criteria of three constraints of optimal design for the constant-pressure cam based on this EUP model were deduced: 1) the pump pressure is equal to the nozzle pressure, i.e., pp ¼ pn ¼ constant; 2) the cam speed should decrease with the increase in injection duration, i.e., 00 c0p < 0; and 3) the cam acceleration gradient should be zero, cp ¼ 0. To verify the optimal designed criteria, the EUP system was tested under different conditions. The experimental results show that the performance of the EUP system is improved by the optimised cam adopting the proposed design constraints. Compared to the tangent cam system, the EUP fuel system with the new constant-pressure cam produces a similar maximum injection pressure of approximately 160 MPa, with a 32% increase in the maximum fuel injection quantity. Furthermore, the optimised EUP system runs at a 19% higher rated engine speed (2500 rpm) compared to the baseline system (2100 rpm). Acknowledgements
Fig. 9. Comparison between the constant-pressure cam and the baseline cam. (a) Peak injection pressure, (b) injection quantity.
We gratefully acknowledge the financial support for this work from the National Natural Science Foundation of China (51006012), the Open Fund of the State Key Lab of Engines in Tianjing University (SKLE K2012-01), the Inner Mongolia Natural Science Foundation (2014MS0501), The Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (CIT&TCD201304038), and the General program of
T. Qiu et al. / Energy 83 (2015) 276e283
science and technology development project of the Beijing Municipal Education Commission (KM201310005033).
References [1] Magno A, Mancaruso E, Vaglieco BM. Experimental investigation in an optically accessible diesel engine of a fouled piezoelectric injector. Energy 2014;64:842e52. [2] Soid SN, Zainal ZA. Spray and combustion characterization for internal combustion engines using optical measuring techniques e a review. Energy 2011;36:724e41. [3] Rakopoulos CD, Dimaratos AM, Giakoumis EG, Rakopoulos DC. Evaluation of the effect of engine, load and turbocharger parameters on transient emissions of diesel engine. Energy Convers Manage 2009;50(9):2381e93. [4] Agarwal AK, Dhar A, Gupta JG, Kim W, Lee CS, Park S. Effect of fuel injection pressure and injection timing on spray characteristics and particulate sizenumber distribution in a biodiesel fuelled common rail direct injection diesel engine. Appl Energy 2014;130:212e21. [5] Bae J, Kim C. A study on integrated design for improving fatigue life of common rail pipe considering stress concentration at complex shape. J Mech Sci Technol 2014;28(9):3617e27. [6] Joachim SH. New developments in MAN B&W four-stroke marine diesel engines. J Inst Eng (India), Part MR: Mar Eng Div 2002;83:5e11. [7] Chaufour P, Millet G, Hedna M. Advanced modeling of a heavy-truck unitinjector system and its application in the engine design process. SAE paper. 2004-01-0200. 2004. [8] Dierre L. Electronically controlled high pressure unit injection system for diesel engine. SAE paper. SAE 911819. 1991. [9] Huang YQ, Leet J. Investigation of in-cylinder NOx and PM reduction with Delphi E3 flexible unit injectors on a heavy-duty diesel engines. SAE paper. 2008-01-1792. 2008. [10] Catania AE, Dongiovanni C, Mittica A. Implicit numerical model of a highpressure injection system. J Eng Gas Turbines Power 1992;114(3):534e43. [11] Hayat Q, Fan LY, Song EZ, Ma XZ, Tian BQ, Farouk N. Nonlinear modelling and analysis of pressure wave inside CEUP fuel pipeline. Math Probl Eng 2014;2014:11. Article ID 521859. [12] Tian BQ, Fan LY, Hayat Q, Ma XZ, Song EZ, Wang H. Computation of pressure fluctuation frequency in electronic unit pump for diesel engine. J Mech Sci Technol 2014;28(4):1529e37. [13] Wang P, Liu FS, Li XR. Driving characteristics for electronic control unit pump fuel injection system. In: 2012 International conference on computer distributed control and intelligent environmental monitoring. IEEE; 2012. p. 264e7.
283
[14] Xu QK, Zhu KQ, Zhuo B. Simulation in thermal design for electronic control unit of electronic unit pump. Chin J Mech Eng (English Edition) 2008;21(5): 1e7. [15] Yu ST, Yang SW, Yang L. Model-based development of real-time software system for electronic unit pump system. Chin J Mech Eng (English Edition) 2007;20(1):25e30. [16] An XH, Liu BL, Cui T, Xie ZX, Zhang FJ. Electronic control unit development for unit pump diesel engine. In: 2010 IEEE International conference on optoelectronics and image processing, Haikou, China; November 11e12, 2010. p. 480e2. [17] Wang TC, Han JS, Xie XB, Lai MC, Henein NA. Parametric characterization of high pressure diesel fuel injection systems. Trans ASME 2003;125:412e26. [18] Hayat Q, Fan LY, Xiu ZM, Bing QT. Comparative study of pressure wave mathematical models for HP fuel pipeline of CEUP at various operating conditions. Int J Smart Sens Intell Syst 2013;6(3):1077e101. [19] Yao C, Fan LY, Dong Q, Song EZ, Tian BQ. Correlation analysis of factors influencing the electronic unit pump cycle fuel injection quantity under overall operating conditions for diesel engines. J Mech Sci Technol 2012;26(9):2929e39. [20] Sundarraman P, Saha SK, Vasa NJ, Baskaran R, Sunilkumar V, Raghavendra K. Modeling and analysis of a fuel-injection pump used in diesel engines. Int J Automot Technol 2012;13(2):193e203. [21] Miura Akinori, yamaguchi Yuhji. Injection rate control of in-line injection pump e cam design through injection process simulation. SAE paper. SAE 950606. 1995. [22] Kegl B. A procedure for upgrading an electronic control diesel fuel injection system by considering several engine operating regimes simultaneously. J Mech Des 1999;121:159e66. [23] Kegl B. Injection system design optimization by considering fuel spray characteristics. J Mech Des 2004;126:703e10. [24] Kegl B, Kegl M, Pehan S. Optimization of a fuel injection system for diesel and biodiesel. Energy Fuels 2008;22:1046e56. [25] Qiu T, Liu XH, Liu FS, Yu L, Gao Z. Research on the performance of EUP fuel system using tangent cam. Veh Engine 2008;179(6):1e3. [26] Cheng W, Cheng HS, Yasuda Y. Wear and life prediction of cam roller follower. SAE paper. SAE 940822. 1994. p. 468e79. [27] Ivantysynova M, Huang CC, Christiansen SK. Computer aided valve plat design e an effective way to reduce noise. SAE paper. SAE 2004-01-2621. 2004. [28] Lindholm P, Bjorklund S. Cortes MCvo. Characterisation of wear on a cam follower system in a diesel engine. Wear 2003;254:1199e207. [29] Mehenny DS, Taylor CM. Analysis of the influence of cam surface waviness on the lubrication of the automotive came and flat faced follower. Tribol Ser 2001;39:863e72. [30] Nayak N, Lakshminarayanan PA, Gaiendra Babu MK, Dani AD. Predictions of cam follower wear in diesel engines. Wear 2006;260:181e92.