RESEARCH Gasoline Engines

18 MTZ worldwide 3/2005 Volume 66

By Gerald Gaberscik,

Christian Lembacher,

Johann Ostermann,

Andreas Rainer and

Michael Trzesniowski

Hochleistungs-Ottomotor

für den Formel-Student-

Wettbewerb

You will find the figures mentioned in this article in the German issue of MTZ 3/2005 beginning on page 210.

High-PerformancePetrol Enginefor Formula Student Racing Cars

In most cases, the different racing formulas stipulate a very nar-row scope for measures permitted on internal combustion en-gines. Especially in cases in which the maximum air flow rate islimited by throttles, it is essential to analyse the requirements tobe met by the powertrain and vehicle very precisely and even tochoose the optimum basic engine. This process, as well as themodification of a series production petrol engine into a high-per-formance throttled engine, was carried out at the Department ofVehicle Technology at the FH Joanneum Gesellschaft mbH. The652 cm3 single-cylinder engine was subsequently used in a For-mula Student racing car.

19MTZ worldwide 3/2005 Volume 66

1 Introduction

The requirements to be met by a racing ve-hicle are defined to a considerable extentby the competition, whereas the tasks to befulfilled in competition dominate the vehi-cle design. This statement is valid for theentire vehicle, and accordingly for both thepowertrain concept and the engine. In thiscase study, the rules stipulate a preciselydefined throttle in the intake tract of a four-stroke petrol engine, which therefore pro-vides the first specification. The competi-tive field of application requires a designthat is adapted for low speeds and narrowcircuits. Thus, a second specification isclearly defined.

High-performance throttled engines areused only for mopeds and in racing [1]. Inmost cases, mopeds are equipped with two-stroke engines, which means that expertisefrom this area can be transferred only to avery limited degree to the four-stroke en-gine. Experience can be transferred, howev-er, from Formula 3. Generally known influ-ences are, of course, useful when it comesto selecting the concept.

Ways of improving the power andtorque of an engine include increasing thedisplacement or increasing the cylindercharge, for example through supercharg-ing. In the present case, the first method isexcluded by the regulations, which meansthat supercharging is the only measurethat is available within the stipulations.The type of supercharger is determinedmainly by a combination of the above-mentioned throttle in the induction tractand the resulting indirect limitation to arelatively low engine speed.

Based on these fundamental considera-tions, the concept was then designed, andthis was followed by the construction ofthe engine based on a standard motorcycleengine. The following targets were definedfor the development of the engine accord-ing to the given application:■ the least possible number of modifica-tions to the basic engine■ distinctive low-end torque characteris-tics■ widely useable speed range■ uniform performance characteristics■ an increase in performance compared tothe basic engine in spite of an air restrictor■ good response characteristics.

2 Concept Design

Due to the precise specifications providedby the rules and the demands to be fulfilledin competition, a matrix was created to aidthe decision-making process, Table 1. Thismatrix shows that four-cylinder and single-

cylinder engines offer the most advantagesfor the intended purpose. The four-cylinderengine bears greater potential compared tothe single-cylinder engine in terms of per-formance, maximum achievable powerand tuning requirements. The special fea-tures of the single-cylinder engine are thelower space requirement, weight, frictionalmean effective pressure and low-endtorque, as well as the costs and good avail-ability of a basic engine that is suitable forthe intended type of supercharging. The ad-vantages to be expected of the four-cylin-der engine are put further into perspectiveby the fact that the maximum achievableoutput is limited by the throttle in the in-duction tract and that the tuning kits avail-able on the market cannot really be used toany good effect.

In order to increase the mean pressureand the power, due to the induction throt-tle above all at low engine speeds, the en-gine was equipped with a supercharger. Asshown in the matrix in Table 2, it was fair-ly easy to design the concept. Due to pooravailability, the Comprex supercharger andthe mechanical supercharger with a flow-type blower were disregarded. The need forthe maximum possible supercharging pres-sure even at low engine speeds was the de-cisive factor in choosing a mechanical su-percharger with a positive-displacementunit.

Once these fundamental considerationshad been made, the 654-R13 single-cylinderengine from Rotax, which is installed in theBMW F 650 GS motorcycle, was chosen asthe basic engine. The M24 positive-dis-placement blower from Eaton was used forsupercharging.

The decision to use an all-wheel drivesystem could easily be made as it offeredclear advantages in the longitudinal andtransverse dynamics of the vehicle. The on-ly disadvantage was the accumulated extraweight. (A description of the conversion toall-wheel drive is not included in this arti-cle.)

3 Conversion of the BasicEngine to the Racing Engine

The engine chosen first had to be adaptedto the maximum authorised displacementof 610 cm3. This reduction in displacementfrom 652 to 597cm3 was achieved by reduc-ing the stroke from 83 to 76 mm. At thesame time, the crank-pin bores in the websof the assembled crankshaft were shifted.The remaining parts of the crankshaft drive, such as the connecting rod and thepiston, remained unchanged. This had theeffect of lowering the geometric compres-sion ratio, as desired, from 11.5:1 to 7.7:1 with

no further effort. The only further measuretaken in the engine/gearbox unit was to re-duce the diameter of the piston fire land by0.2 mm, and to add pressure-relief grooves.This allowed the engine to withstand thegreater thermal expansion caused by thehigher combustion chamber.

The basic set-up of the induction line,Figure 1, was due in part to the competi-tion rules. These stipulate the sequence ofthe throttle unit – air restrictor – super-charger – engine in the flow direction. Fur-ther design steps were initially taken usingone-dimensional flow calculation (tool:AVL boost) and these were then imple-mented according to the space require-ments in the vehicle. The use of a charge-aircooler was defined as a result of these testcalculations. It not only achieves an in-crease in torque conversion of around 7 %at lower engine speeds, but also providesan advantageous stabilising volume be-tween the continuously conveying super-charger and the intermittently aspiratingsingle-cylinder engine. The influence of thelength of the conduction lines between thesupercharger and the charge-air cooler, andbetween the cooler and the combustionchamber, was in each case so minor thatoptimisation in this area was not undertak-en. It was also practically impossible due tothe lack of space in the vehicle.

Another idea, which was already elimi-nated in the concept phase after simulationcalculation, was to use double injection. Inprinciple, it was to inject part of the fuel inthe area of the throttle valve, thus in frontof the air restrictor. The theoretical advan-tage of evaporation cooling of the combus-tion air is compensated by the modifiedvolume flow in the supercharger (air plusfuel instead of pure air), and the charge inthe combustion chamber hardly changes.Another argument against double injectionis the danger of icing at the narrowestcross-section of the air restrictor.

The supercharger, Figure 2, is drivendirectly by the generator crankshaft via aribbed V-belt (type Poly-V from Hutchin-son) with a gear ratio of i = 1.7. The belt drive offers greater freedom of choice of thecentre distance, and, due to its slippingtransmission behaviour, it smoothens thenon-uniform torque output of the single-cylinder engine. The drive pulley is screwedto the crankshaft via a hub instead of thegenerator rotor. This hub simultaneouslysecures the face for the shaft sealing ring.The standard engine cover was reworkedso that it enabled the shaft end to protrudeand to include the gasket ring attachment.

Further measures are generally requiredon the basic engine during superchargingto compensate for the increased mechani-

RESEARCH Gasoline Engines

20 MTZ worldwide 3/2005 Volume 66

cal and thermal loads caused by the higherpower. Such measures were not taken inthis case. Compared to the basic engine in astandard motorcycle, a significantly shorterlifespan was considered sufficient for use ina racing vehicle. Furthermore, the enginedimensions were not to be increased as a re-sult of this application. What is more, thebasic engine is already cooled by sprayingthe piston crown with oil. In relation to theincreased engine power, this may lie at thelower limit of tried-and-tested cooling-oilvolumes, but was still just sufficient so that,in the end, no modification was required.

Due to their basic principle – the super-charger is designed for the full-load air vol-ume of the engine – mechanical super-chargers require a boost-pressure control.The obvious approach of not running thesupercharger at partial load was not fol-lowed up. This solution entails high costsand involves development problems. More-over, due to its application, the partial-loadbehaviour of a racing engine is of minor im-portance. The permanent drive of a super-charger without internal compression, as inthe case of the Eaton supercharger, is alsoacceptable from an energy point of view. Asolution known from standard applicationswas carried out via a supercharger bypassline. At low partial load, the supercharger(4, Figure 1) would be operated outside itspractical map area. As a result, the enginedraws in the combustion air via the bypassline. A bypass or non-return valve (6) isopened by the induction-pipe vacuum. Thisvalve closes with increasing superchargingpressure and the engine receives the com-bustion air from the supercharger. The ex-cess air from the engine at high enginespeeds and loads is again fed to the super-charger via the bypass line.

Special attention was paid to the designand integration of the stipulated air restric-tor with a 20 mm diameter throttle,through which the entire combustion-airvolume has to be drawn. Such a throttle po-sition limits the maximum air mass flowand thus the peak power and the maximumrotational speed of an internal combustionengine. The theoretically possible mass

flow through a throttle position in givenboundary conditions such as environmen-tal pressure, density of the environment airand adiabatic exponent of the air is propor-tional to the surface of the narrowest cross-section [2]. In this case, the theoretical airflow rate is limited at 0.075 kg/s, whichmeans that the theoretical maximum rota-tional speed of the engine with a displace-ment of 597 cm3 is limited to 11,600 rpm.

The air restrictor is installed in the con-necting tube between the throttle poten-tiometer housing and the supercharger in-let, Figure 3. The air-ducting cross-sectionis provided by a Laval jet. The first conver-gent sector accelerates the air flow to theactual restrictor. This is followed by a dif-fuser that simultaneously creates the tran-sition from the circular narrow point to theflat, oval supercharger inlet. The super-charger bypass line leads into the connect-ing flange of the supercharger.

The fuel volume is injected via two in-jectors. Each injector is in a separate portbranch and injection occurs in front of theinlet valve of the forked induction port. Itwas not possible to cover the injection vol-ume sphere with the required metering ex-actitude using a single injector.

A control unit (12, Figure 1) is responsiblefor the entire control of the engine. Thisunit processes the input values of a throttlepotentiometer (13) as a load signal and a ro-tational speed signal (14) from the crank-shaft, which simultaneously returns thetop dead centre (TDC) signal. This effective-ly means that the control does not differen-tiate between a charge TDC and an ignitionTDC. From these input values, the controlunit calculates the signals for the injectorsand the ignition output stage. Via furtherinput of pressure (16) and temperature (15)in the induction tract after the charge-aircooler (5), corrections are undertaken for thefinal fuel-injection volume and the ignitionpoint. The temperature of the coolant (17) isused for controlling the warm-up enrich-ment. The control device also switches onthe electric fan (19), which ensures a posi-tive flow-through ventilation of the heatexchanger in the engine cooling system.

4 Results

The most impressive result is representedby the comparison to the basic engine witha larger displacement, Figure 4. The adapt-ed engine with a supercharger already pro-vided a torque of 70 Nm from 2500 rpm.That represents a mean effective pressureof 14.7 bar. The further curve of the full-loadcharacteristic line is extremely flat andachieves its maximum of 80 Nm torque at5000 rpm. The mean effective pressure hereis 16.8 bar. The crank train, however, is sub-ject to a significantly higher load becausethe subsequent average supercharger pow-er output of 5 kW (equal to BMEP meanpressure = 2 bar) is also provided by the en-gine. The supercharging pressure reaches1.8 bar. Despite the simple by-pass control,the vehicle has good driveability. This isachieved not least by the engine control,which not only improves acceleration, butalso reduces the injection volume depen-dent on the closing speed of the throttlevalve. As a result, the driver perceives nodelay in the throttle response or during thetransition to shift operation, although themean air travel between the throttle valveand the induction valves is around 1450mm.

5 Summary

This example shows that, by using me-chanical supercharging, a high-perfor-mance throttled engine can take the formof a four-stroke single-cylinder engine. Theextent of the modification of the basicpetrol engine with a displacement of 652cm3 and the additional equipment could bekept to a minimum without adverse effectson driveability. All of the set targets – amarked low-end torque characteristic, awidely useable speed range, a uniformpower development, increased power com-pared to the basic engine despite the use ofan air restrictor and less displacement, andgood response – could all be achieved at thelowest possible modification costs. Such anengine from a motorcycle was designed atthe Department of Vehicle Technology atthe FH Joanneum Gesellschaft mbH andsuccessfully used in a Formula Student rac-ing vehicle.

RESEARCH Gasoline Engines

3 Conversion of the Basic Engine to the Racing Engine

Figure 3: Induction route of the air restrictor, partly cut

References

[1] Gaberscik, G.: Untersuchungen an einemFormel-3-Motor. Conference IAT ´95, Radein,Slowenien, 1995

[2] Indra, F.; Grebe, U. D.: Der Formel-3-Renn-motor von Opel. In: MTZ 54 (1993), Nr. 11,S. 576–584