Keywords: engine tuning, flow coefficient, mathematical model, camshaft Abstract This article deals with the tuning of a mass-produced engine Skoda 781.136B and its rebuilding into a racing engine. The introduction briefly describes the basic parameters of the mass-produced engine. The information is then followed by a detailed description of adjustments to the pipe system, valve timing, cylinder head and crank mechanism. The article presents the benefits in terms of increasing power parameters and there is also a comparison of speed characteristics. The aim of the tuning was to increase the engine power parameters, in particular the torque in the range of 4 000-6 000 min -1, at which the engine most often operates during competitions. The adjustments and optimization of the engine have increased the power parameters in the required range of revolutions by 38-47%. 1 INTRODUCTION When the tuning power parameters of a racing engine are considered, fuel consumption is not a critical parameter, since the consumption is often three times higher than that of the massproduced engine. In racing cars the overall life of the engine significantly decreases, however, maximum levels of power and torque are achieved. The tuning modifications may also contribute to the development of new, more powerful engines for mass production. In racing car engines, emission regulations need not be complied with. This article deals with adjustments to the engine Skoda 781.136B that is to participate in the race KW Berg - Trophy (uphill race). The aim of the adjustments is to increase the power parameters in the speed range 4000-6000 min -1. The engine, with 1289 cm 3 displacement, belongs in the sports category E 1400 [1]. 2 SKODA 781.136B ENGINE CHARACTERISTICS The engine Skoda 781.136B was assembled into the mass-produced series Skoda Favorit and Skoda Felicia. It is designed as a water-cooled inline four-cylinder engine, with OHV valve distribution system. The crankshaft is mounted on three sliding bearings. The cylinder head is produced of aluminium alloy; it has eight channels and four roof-shaped combustion chambers. In contrast to the previous model of this engine, the pistons have a flat bottom that has increased the compression ratio from 8.5:1 to 9.7:1. The ignition is controlled by a distributor with centrifugal and vacuum regulation. The engine lubrication is provided by a gear pump driven by the camshaft. For a detailed description of the engine see [2]. 1 2,3 Slovak University of Technology in Bratislava, Faculty of Mechanical Engineering, Námestie slobody 17, 812 31 Bratislava, branislav.ragan@stuba.sk 1, marian.poloni@stuba.sk 2, andrej.chribik@stuba.sk 3
3 MATHEMATICAL MODELLING OF THE MASS-PRODUCED ENGINE Figure 1 Mathematical model of the mass-produced engine Skoda 781.136B 1 throttle, 2 air filter, 3 intake manifold, 4 intake port, 5 intake valve, 6 cylinder, 7 exhaust valve, 8 exhaust port, 9 exhaust manifold, 10 catalytic converter, 11 silencer, 12 fuel vessel Figure 2 Course of valve openings (above) and course of flow coefficients (below) l [mm]- valve stroke, d [mm]- valve seat diameter, TDC top dead centre, BDC bottom dead centre, [ ]- angle of crankshaft, Cf [-]- flow coefficient,, ivt intake valve tuned, ivo intake
valve original, evt exhaust valve tuned, evo exhaust valve original, ipo intake port original, ipt intake port tuned, epo exhaust port original, ept- exhaust port tuned In order to be able to consider a greater variety of treatments in a relatively short time and their effects on engine parameters, it was necessary to compile a rather precise mathematical model of the engine in the Lotus Engine Simulation (LES) programming environment. Besides other parameters, the mathematical model allowed for optimisation of the geometry of the pipe system and the valve timing for the required speed range. As for the preparation of the mathematical model, it was necessary to collect as many parameters of the actual engine as possible, to disassemble the engine and to measure all the information as required by the LES program. The LES program helped model the mass-produced engine in great detail, as can be seen in Fig. 1. To obtain the most accurate results based on the modelling, and also the measuring of the geometrical dimensions of the crank and valve mechanisms, it was necessary to measure the valve timing and the flow coefficients of the inlet and outlet ports [3]. The measured values are shown in Fig. 2. 3.1 COMPARISON OF THE MODEL AND EXPERIMENT After the input of all parameters into the LES program, the speed characteristic of the mass-produced engine was calculated. Fig. 3 shows the modelled speed characteristics compared with those taken from the actual engine as measured on a dynamometer. Figure 3 Comparison of speed characteristics from the mathematical model with those from the experiment M t [N.m] - torque, P [kw]-effective power of the engine, n [min -1 ] - engine revolutions, Mt b - torque -brake, Mt m - torque -model, P b power-brake, P m power-model The differences in the values of power do not exceed 1% (0.47 kw) and the values of torque do not exceed 2% (1.94 N.m). The considered mathematical model was then gradually altered in dependence on the modifications made to the actual engine. Based on the recent results, good
agreement between the experiment and the mathematical model can be assumed, even for the tuned engine. 4 ADJUSTMENTS OF THE MASS-PRODUCED ENGINE 4.1 CRANK MECHANISM The pistons with 75.5 mm bore were replaced by larger ones, with a 77 mm diameter, which resulted in increased engine capacity of 1341 cm 3. The pistons were partially mass-relieved to reduce the inertia forces of the sliding materials. The pistons together with piston rings and the piston pin were balanced to the nearest 0.5 gram. Shanks of connecting rods were finished to mirror lustre in order to increase resistance against notch fracture. Similar to the pistons, the connecting rods were balanced (rotating and sliding part) to the nearest 1 gram. 4.2 CYLINDER HEAD The cylinder head underwent intensive adjustment. By grinding the bottom of the head, the compression ratio increased from 9.7:1 to 11.3:1. The increased compression ratio led to increased thermal efficiency of the engine. The intake and exhaust ports were enlarged by special cutters and grinding wheels and were accurately joined onto the exhaust and intake manifolds, thus increasing the filling efficiency of the engine, as well as reducing pumping losses. The combustion chambers were cut and smoothed to increase resistance against carbonation of the combustion chamber, thus increasing resistance against possible detonation and self-ignition. 4.3 CAMSHAFT Increasing the filling efficiency and a shift in power parameters to higher speed affects the timing of the camshaft. By optimising via the LES program, the best possible course and angle of opening and closing of the intake and exhaust valves (Fig. 2) were designed in. In agreement with the modelled timing the mass-produced camshaft was reground. 4.4 INTAKE AND EXHAUST MANIFOLDS In the same way as the valve timing, the intake and exhaust manifold were optimised by the LES program in such a way as to get the maximum torque and power out of the engine in the speed range 4000-6000 min -1. After the shape and pipe lengths were designed, the piping was welded from tubular profiles. As regards the arrangement of the branch binding, the exhaust manifold was designed in two types of construction: 1. 4-1 (i.e. four exhaust pipes from each cylinder into a single output pipe) 2. 4-2-1 (i.e. four exhaust pipes from each cylinder are first connected to two pipes, and these are subsequently combined into a single output pipe). Both types of construction of the exhaust pipe system were modelled with the LES program in order to assess their impact on the engine power parameters. The exhaust arrangement 4-1 provided increased power (by 2 kw) at maximum speed, as compared with the arrangement 4-2-1, however, the increase of torque in the desired speed range was higher (by 17 N.m) with the exhaust 4-2-1 (Fig. 5). Therefore, we decided to manufacture the exhaust manifold with this arrangement. [4]
5 COMPARISONS OF MASS-PRODUCED AND TUNED ENGINE CHARACTERISTICS After all adjustments to the mathematical model in the LES program, the speed characteristic was compared to the speed characteristic of the mass-produced model (Fig. 4). Figure 4 Comparison of the calculated characteristics of mass-produced and tuned engine M t [N.m] - torque, P [kw]-effective power of the engine, n [min -1 ] - engine revolutions The point of maximum torque has been shifted from the original 3750 min -1 to 4500 min - 1. Its maximum value has increased from 100 N.m to 147 N.m. In the operating speed range, for which the engine has been designed, the torque curve is smooth and significantly higher than the torque of the original engine. The maximum power has increased from the original 50 kw to 78 kw at 6000 min -1. 6 CONCLUSIONS The aim of the tuning was to modify the engine used in mass-produced cars to fit racing car requirements, so that the engine could participate in the KW Berg-Trophy race. With the LES the piping system and the valve timing were optimized, which had a positive impact on increasing the filling efficiency and thus the power parameters of the engine. Other adjustments described in the article led to increased thermal efficiency and reduced mechanical losses. In the tuned speed range, the filling efficiency increased by 13%. This was a positive contribution to the increase in torque, from 100 N.m to 147 N.m, and in power, from 50 kw to 78 kw.
Figure 5 Photos of the mass-produced (left) and the tuned engine Skoda 781.136B (right- exhibited at Machinery Fair in Nitra, Slovakia). REFERENCES [1] J. Danko, Ľ. Magdolen, T. Milesich: Modelling of the damper characteristics of the unmanned ground vehicle, Transport means 2013, ISSN 1822-296x, Kaunas, Lithuania, 2013 [2] R. Cedrych: AUTOMOBILY ŠKODA FELÍCIA, Grada Publishing Praha, 2000, ISBN: 80-7169-718-4. [3] Lotus Engineering: Lotus engine simulation (LES), User s Guide Version 5.0.6., Hethel, Norwich, U.K. [4] Grznár M., Gavačová J., Possibilities of vehicle design solutions in new requirements, 18th International Conference of Transport Means 2014, Lithuania: Kaunas University of Technology, 2014, p.428-430. ISSN 2351-4604 ACKNOWLEDGEMENTS This work was supported by the Slovak Research and Development Agency under Contract-No. APVV-0015-12 and was also supported by the Scientific Grand Agency under Contract No. VEGA 1/0017/14.