Cylinder Pressure Based Calibration of a Formula SAE Racing Engine Gabriel Tatsch, Mario Martins, Thompson Lanzanova, Rafael Lago Sari, Cassio GĂśrck Federal University of Santa Maria
ABSTRACT In internal combustion engines, cylinder pressure cyclic variations lead to torque fluctuations, which have large influence on vehicle control and engine fuel consumption two very important aspects of a Formula SAE race car. In order to improve fuel consumption and vehicle driveability this paper describes the test bench results of cylinder pressure based engine calibration used for the Formula UFSM racing team. Using the pressure signal provided by a piezo sensor located inside the combustion chamber to perform combustion and heat release analysis, the objective was to reduce the combustion variability at low speeds and low loads, conditions which were defined as critical when analyzing track logged data from previous competitions.
INTRODUCTION The project and development of high performance engines to be used in automotive competitions were always a big challenge for the internal combustion engines designers. The engine must provide high output with maximum fuel efficiency. Serviceability and durability also play an important role on such events, which have specific rules to be followed. Torque and power and the way they are delivered to the powertrain will influence directly the driveability of the vehicle, as well as the throttle response at part load conditions [1, 2]. Throttle response and engine load control are very important due to the track characteristics on Formula SAE (also known as Formula Student) competitions. They have a few straight zones and many curves. At the corners the driver should balance the throttle to maintain grip with the tire and the track surface, thus engine acceleration after apex zone should be smooth and torque fluctuations must be avoided. Cylinder pressure cyclic variations are harmful for vehicle control and increase fuel consumption, as a consequence of torque fluctuations. This paper aims to present test bench results of cylinder pressure based engine calibration of a four cylinder motorcycle engine modified to run on a Formula SAE race prototype. The Formula SAE competition exists since the 70s and it is a student engineering competition which aims to prepare the student for the market, especially in the automotive industry. It is a well-known competition (figure 1) which consists of static and dynamic events and also challenges a group of students to hypothetically establish themselves as a small business which has to design, manufacture and test an openwheel single-seater race car prototype. The students have also to evaluate the vehicle’s potential as a production item, all bordered by a regulation made by the organizing committee of the competition [3]. In Brazil, the competitions exists since 2004 and each year the number of participants has been increasing. The Formula UFSM Racing Team was founded in 2010 and, despite of its short existence it is now among the best teams in Brazil.
Figure 1 – Formula SAE/Formula Students competitions around de world
The combustion process in spark ignition engines is sensitive to cycle-to-cycle and cylinder-to-cylinder variations. These are the results of differences in burning velocity, charge motion and mixture composition around the spark-plug when the electric discharge happens [4]. As the calibration is made for an average cycle, these variations should be as small as possible, and a compromise between the average air-fuel ratio and spark timing should be found to achieve the optimal engine performance. Furthermore, large cycle-to-cycle variations limit the engine operation range and are responsible for torque variations which affect vehicle driveability [5]. Sport motorcycle engines normally use a short stroke compared to piston bore. This constructive characteristic is due to the need to operate at high speeds and achieve a high specific power. On the other hand at low speeds and loads combustion tends to be unstable [6]. The most common index to measure combustion stability is the covariance of the indicated mean effective pressure, which is the standard deviation of the IMEP divided by the average for a certain number of cycles. It is possible to ensure that beyond 10% of COV IMEP, vehicle driveability is affected [5] Charge motion within the cylinder plays an important role on combustion process control, since their behavior are determinant for mixture formation and heat transfer. The characteristics of a tridimensional and turbulent flow have origin from the intake ports and can suffer some modifications at the moment they enter the combustion chamber due to the piston movement and its geometry [5]. There are two main in-cylinder charge motion patterns: swirl and tumble. The first one is defined as the flow movement around the vertical cylinder axis and the second one as the swirled movement around the horizontal axis of the cylinder [7]. These movements are used as a way to increase the engine thermal efficiency, reduce fuel consumption and exhaust emissions. This benefit comes from the increased burning rates allowed by these two motion patterns, which allow the extension of the lean mixtures operation limits and the use of high compression ratios. Nevertheless, there are some drawbacks such as the increased heat exchange with the cylinder walls and reduced volumetric efficiency. [8, 9, 10, 11, 12] The use of electronic fuel injection on spark-ignition engines has as the advantage to increase the volumetric efficiency and to evenly distribute the fuel to the cylinders. In general, with PFI injectors, the characteristics of an homogeneous and external mixture formation leads to small interaction with the fuel and the intake manifold walls, reducing the wall film formation [13] Kato et al. (2008), aiming to reduce fuel consumption, conducted a study about the influence of a PFI injection on combustion stability. It was used a short stroke motorcycle engine based on a production model. 4 different injection systems were investigated, with different injection direction, injection timing and fuel pressure. It has been found that even with a PFI injection system mixture distribution in the cylinder is affected. Systems with wall film formation tend to adversely affect engine performance under transient operation [6]. Following the same line of the previous research work, Youmoto et al. (2011) used methods of visualization of the fuel spray, fuel film and initial flame propagation in various settings of a PFI injection. It was concluded that the combustion stability as well as its duration is affected by the conditions of homogeneity of the mixture within the combustion chamber. It is mainly due to the formation of a liquid fuel film on the walls of the duct and the size of the fuel droplets formed. In low load conditions is it possible to inject small droplets, since they are not many and small, without affecting the combustion stability. Spray less than 30 microns SMD proved great improver in achieving a stable combustion at low load conditions and enable a good throttle response [14]. Pontoppidan and Baeta (2013) used an aggressive engine downsizing, 4-cylinder with 1.4 L. It was found that at low speeds and low loads the original PFI injectors did not produce the expected performance and fuel consumption. Through computational numerical simulation has been found that an atomized injector with 6 holes allow better mixture formation. Experimental tests showed that there was a reduction in consumption up to 14 % without penalizing engine performance. [15] The combustion stability is described by Merker et al. (2012) as the covariance of the indicated mean effective pressure (COVIMEP). Parameters as combustion duration, from 10% to 90% of mass fraction burn, as well as the initial flame front formation, until 10% mass fraction burn, are important on the combustion evaluation and heat release analysis. [7] In most of the cases the combustion instability is directly linked to the initial flame formation, which is most susceptible to variations on flow field velocities and mixture inhomogeneity. Nowadays combustion engines researches try to reduce fuel consumption and exhaust emissions to enter in the regulation, and as much as possible to maintain or increase performance. One of the strategies is operate the engine at lean mixture. According to Aleiferis et al. (2000) this system presents issues on cyclic variability and flame stability at the beginning of the combustion. [16] Young (1981) verified that eliminating the cyclic variability it is possible an increase up to 10% on engine power without any increase on fuel consumption [17] The first step to develop an engine to be used on Formula SAE competitions is to follow the rules. Among several rules imposed by the rule, those which are most relevant for this work are: [18]  
The engine must be four stroke with maximum 610 cmÂł of volumetric displacement; It must have an air restrictor of 20 mm for gasoline or 19 mm for E85, in the intake manifold upstream the throttle body;
All the air which enters in the engine must pass through the throttle body and the air restriction, electronic devices as Drive-by-Wire are forbidden; The engine should not exceed 110 dBA; All the engine parts should be inside an envelope determinate by the Main-Hoop and the external part of the tyres.
Ceviz (2006) verified that the Airbox influence not just on engine performance but on combustion and emissions. As much as bigger the airbox, higher the pressure inside the intake manifold and leaner the mixture. There was a decrease on COVIMEP and CO and HC emissions. The advantages using a big volume Plenum on static conditions it is not the same for track situation where the throttle responses are damaged [19]. Engine calibration is very important in function of the many variables involved on engine development process. At the same time the engine should attend emissions regulations, been efficient, have high durability and has high output. Racing engine calibration is a determinant factor to fulfill the whole operation range. Westin and Angström (2005) calibrated a Formula SAE engine using DoE and 1D simulation to achieve maximum power [20]. Modern systems as presented by Müller et al. (200) used in cylinder pressure data to minimize fuel consumption and exhaust emissions. The ECU controls the parameters aiming to decrease cyclic variability and maximize torque, which performs better vehicle driveability without knock occurrence [21].
EXPERIMENTAL SETUP ENGINE The engine in which the experiments were carried out in this paper is originally from a Honda CBR 600 RR sport motorcycle. Intake, exhaust and injection systems were modified to fulfill the competition rules. The principal constructive and operational characteristics are listed at table 1. Model Number of cylinders Valves in each cylinder Bore x Stroke Volumetric displacement Compression Ratio Intake valve diameter Exhaust valve diameter IVO IVC EVO EVC Firing order Injection system ECU Fuel Air restriction diameter
Honda CBR 600 RR 4 in line 4 67 x 42,5 mm 599 cm³ 12,2:1 27 mm 22 mm 21° BTDC 44° ABDC 40° BBDC 5° ATDC 1-2-4-3 PFI Motec M800 Petrobras Podium 20mm Table 1 Engine characteristics
The intake manifold demonstrated by the figure 2 as well as the exhaust manifold in figure 3, were determinate through computational 1D CFD simulation on GT-Power. The intake manifold has:
Throttle Body: commercial with 34mm adapted; Air Restrictor: 20 mm to run gasoline; Plenum: 3 liter in aluminum; Runners: 250 mm length and 31,75 mm diameter aluminum
The muffler was acquired as sponsorship and the other pipes were determined also through computer simulation and were made of steel. It was used one fuel bench with PFI injectors, which provided a fuel flow of 2.15 g/s pointing directly to the intake valve head. The electronic control unit used is the Motec M800, which allows launch and traction control strategies. Furthermore, it works as datalogger which enables to save data from the engine and vehicle sensors such as to be analyzed after the testing events and the competition itself.
Figure 2 - Intake Manifold
Figure 3 - Exhaust Manifold DYNAMOMETER SETUP In this paper, a manually controlled hydraulic dynamometer was used to run the tests, as shown in figure 4. Torque was measured by a strain gaged load cell and data were acquired with a National Instruments board NI 6015.
Figure 4 - Engine run on dyno
The sensors in the engine can be divided in two groups: sensors connected on the ECU and the ones connected to two acquisition boards from National Instruments. The main sensors are illustrated in figure 5. On the right side are listed the sensors connected to the ECU and, on the left, there are those which are connected to the acquisition boards. The instantaneous engine fuel consumption was measured using a graduated burette with 100 ml and resolution of 1 ml. The fuel consumption of 30 ml was timed and recorder for all tests.
Figure 5 - Main sensors in the engine PRESSURE SENSOR INSTALATION Cylinder pressure was acquired using a piezoelectric sensor installed in cylinder number one, between intake and exhaust valves. An incremental encoder was used for referencing to the angular position. The sensor used was an AVL GH 14 D model. A sleeve mount was necessary because the hole passed through the water jacket. The cylinder head was drilled with an angle of 35째 and tapped internally to maintain the sensor tight and to avoid any water leakage inside the combustion chamber and outside the engine.
Figure 6 - Pressure sensor sleeve position
TESTS PROCEDURE After warm-up the engine was set to run in a stable low load operation point. Four part load operating points were analyzed. These points were selected from track logged data from the Formula SAE West competition in 2012. By reading the logged data it was possible to conclude that the driver had almost no throttle control during the dynamic events, as shown in figure 7. This is a result of the modifications made in the intake and exhaust manifolds to satisfy the rules and a non-adequate engine calibration at low speeds and low loads.
Figure 7 – Logged data There are three series of data on the logged event. The first group consists of three curves provided by the suspension travel sensors installed on the vehicle. It should read four curves, one for each shock absorber. However one of them had an issue and the sensor was not reading properly. The green line in the middle of the figure 7 expresses throttle position (in percentage) and last curve shows engine speed in revolutions per minute. By using the suspension travel sensor signals it is possible to verify when the driver was turning, entering and leaving a corner. This paper focused specifically on these engine map points. When cornering, the driver would normally go off the throttle and brake prior from entering the corner and would go on the throttle again by the time the car would reach the apex. Therefore, the driver should carefully modulate the throttle and the brakes in order to maintain the grip and do not lose time with deceleration. What it possible to ensure from figure 7 is that the driver doesn’t have ideal engine control during great part of the circuit due to large throttle variations. When going on the throttle again, it is possible to verify that the engine speed was not rising smoothly, which means that the torque curve is either not adequate or the calibration is not properly done to provide the driver a vehicle control as it should. To have a smooth transition between idle and part load in small stroke engines is a big issue to be solved and through a suitable engine calibration conditions could be improved. The experimental tests were carried at 3500 and 5250 rpm. For each of engine speed two part load operating conditions were used according to the throttle position. The tests were performed at the operating points listed at the table 2. Engine Speed (rpm) 3500 3500 5250 5250
Throttle Position (%) 26 35 20.5 29 Table 2 - Operating points
A Labview routine was created to process the series of data provided by the cylinder pressure sensor for sets of 200 cycles each. During the test, the engine was kept at constant speed and throttle opening while the mixture was modified. Three lambda values were normally used: rich, stoichiometric and lean conditions. After that, it was verified at which conditions the combustion showed the minimum value for COVIMEP. After the mixture was fixed, spark timing was modified to find a compromise between torque, brake specific fuel consumption and COVIMEP. Figure 8 shows the part load conditions on the engine map.
Figure 8 – Operating points at part load
RESULTS Without any modification on the previous map, with the engine at 3500 rpm, 24% throttle and ignition timing fixed in 25 degrees before top dead center, the lambda reading was 0,93 and the engine would produce18,18 Nm brake torque and COVIMEP higher than 10%. When lambda value was changed, it was found that a value of 0,98 produced a COVIMEP smaller than 10%, which means a better performance in what concerns vehicle driveability. By advancing the spark timing it was possible to verify a reduction in combustion variability and an increase in brake torque, achieving MBT at 35° BTDC. The cyclic variability tends to decrease with the ignition advance set to 40° BTDC, following a torque reduction and increased fuel consumption Thus, the optimum operating point was with lambda 0,98 and ignition timing set to 35°BTDC, as shown by figure 9 a) and b).
b)
600
30
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500 450 400
20 15 10 5
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Figure 9 – 3500 rpm and 24% TPS a) BSFC x COVIMEP; b) TORQUE x COVIMEP
Keeping the engine speed constant at 3500 rpm and throttling until 35%, the best result was with lambda 0,94, showing a COVIMEP of less than 5%. Likewise, advancing the spark timing would increase brake torque increase while reducing COVIMEP. Figure 10 a) and b) express the best relation between BSFC, torque and combustion variability, which was found with ignition timing of 45° BTDC.
b)
340
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320 310 300 290
34 33 32 31
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Figure 10 – 3500 rpm and 35% TPS a) BSFC x COVIMEP; b) TORQUE x COVIMEP Rising the engine speed to 5250 rpm and closing the throttle to 20,5% showed an excess of fueling, leading to high BSFC values of up to 420 g/kWh. Regarding combustion stability, however, COVIMEP reached values of of 6,5% for a torque of 17,9 Nm. By leaning out the mixture close to the stoichiometric value, fuel consumption decreases, torque remains almost the same and a small reduction on COVIMEP is noticed. However, combustion variability reached its minimum for a lambda value of 0,92. By adjusting the ignition timing it was possible to reduce COVIMEP while torque had a small reduction as well as fuel consumption, which compensate for this torque reduction. This is expressed in the figure 11 a) and b).
b)
440
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420 410 400 390
18 16 14 12
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Figure 11 – 5250 rpm and 20,5% TPS a) BSFC x COVIMEP; b) TORQUE x COVIMEP
8
Finally, as shown the figure 12 a) and b) for the last operation point in this study, the engine speed was maintained at 5250 rpm and the engine loaded to 29% TPS. Again, the best value for combustion stability was reached with lambda slightly enriched. Torque and fuel consumption achieved satisfactory values, respectively, 32,66 Nm and 317,88 g/kWh. Even with COVIMEP at less than 5% it was possible to improve combustion stability despite a small decrease in torque although with a gain on engine brake specific fuel consumption.
b)
420
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Figure 12 – 5250 rpm and 29% TPS a) BSFC x COVIMEP; b) TORQUE x COVIMEP
CONCLUSION This is study showed the use of cylinder pressure data as an important alternative to improve engine calibration for Formula SAE racing teams. Using the piezoelectric sensor to acquire in-cylinder pressure signal and a Labview routine to process this signal to provide COVIMEP values made possible to:
Achieve less than 5% COVIMEP in all part load operating points tested; Increase torque and decrease BSFC; Ensure that despite the small stroke engine characteristics it is possible to achieve a stable combustion at low speed and load conditions without using any charge motion device;
As this was the first study regarding combustion analysis performed by the Formula UFSM Racing Team, some future work could be performed such as:
To use this method to calibrate the entire engine map on test bench and track tests; To enhance the analysis by using computer simulation with a validated model to show heat release and combustion duration studies.
ACKNOWLEDGMENTS The authors would like to thank the Internal Combustion Engines Research Group from Federal University of Santa Maria for all the support and for allowed the use of the facilities where the tests took place. In addition, the authors would like to thank Formula UFSM Racing Team which kindly lended their engine for the test.
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CONTACT Gabriel Tatsch – tatsch16@gmail.com -+5555 9109 7881. Mario Martins – mario@mecanica.ufsm.br + 5555 9622 2373
ABBREVIATIONS 1D ABDC ATDC BBDC BDC BSFC BTDC CFD CO COV DoE ECU HC IMEP LVDT PFI SAE TDC UFSM WOT
One-Dimension After Bottom Dead Center After Top Dead Center Before Bottom Dead Center Bottom Dead Center Break Specific Fuel Consumption Before Top Dead Center Computational Fluid Dynamics Monoxide Carbon Covariance Design of Experiments Eletronic Control Unit Hidrocarbon Indicated Mean Effective Pressure Linear Variable Displacement Transducer Port Fuel Injection Society of Automotive Engineers Top Dead Center Federal University of Santa Maria Wide Open Throttle