Novel VCR Mechanism Analysis using Honda Engine
5.1 NOVEL VCR RESULTS USING HONDA ENGINE
5.1.2 Effect of speed on performance parameters with novel VCR mechanism
It is well acknowledged fact that an internal combustion engine can operate at the highest possible CR for attaining the highest overall efficiencies. But, the gain in efficiency beyond a certain CR can be expected to be marginal due to other influencing factors such as heat loss and friction (Martyn, 2002). It has been experimentally investigated that upper limit of compression ratio for SI engine is very specific to a particular engine for which there is fall in efficiency after certain CR for iso-octane fuels (Sridhar et al.,2001). This particular information is evident with this VCR mechanism about the chosen engine. The comparison of engine parameters between original engine and the modified engine attached with the new VCR mechanism at no load condition, compression ratio of 4.8 and fixed throttle position of 52% are given in Table-5.2. For both fuels, it is found that the brake power of the modified engine is same as that of the original engine. However, increment of 1.3% and 9.7 % has been observed in the brake thermal efficiency (BTE) for petrol and kerosene respectively,
with the engine modifications. Similarly, the brake specific fuel consumption (BSFC) for petrol and kerosene with the incurred changes to accommodate the VCR mechanism showed slight decrement of 1.1 % and 9.5%, respectively. The reason for this disparity is the uncertainty in measurement and subsequent machining of the modified engine head. Studies are extended with the base engine setting along with VCR attachment to analyze the engine performance for change in engine speed. Further, the presently designed VCR mechanism has been tested for different compression ratios with minute increment above the original compression ratio (CR) of 4.8. In these experiments, fuel being used is petrol having research Octane number 91 (RON-91) and kerosene (RON-75).
Table 5.2.Performance parameters of the engine with modified cylinder head at no load condition
Fuel Performance
parameter Original head
New head with VCR mechanism
Deviation (%)
Gasoline Torque (N-m) 0.28 0.28 0
Brake power (W) 64 64 0
BTE (%) 1.52 1.54 1.32
BSFC(kg/kWhr) 1.5 1.48 1.07
Fuel consumption
(cc/sec) 0.037 0.036 2.77
Kerosene Torque (N-m) 0.093 0.093 0
Brake power (W) 21 21 0
BTE (%) 0.44 0.48 9.7
BSFC(kg/kWhr) 5.01 4.53 9.54
Fuel consumption
(cc/sec) 0.038 0.034 10
The variations of engine performance parameters with speed (or load) are plotted in Figs. 5.2 and 5.3. During testing, the engine has been set at 52% wide open throttle (WOT) position.
At this throttle position, for the knock limited speed of engine (obtained by applying maximum possible load), the data for net brake force, fuel consumption, air consumption, speed have been recorded and the corresponding trends are plotted (Figs. 5.2 and 5.3). In the successive investigations, the set of data has been collected with constant increment of 100 rpm in speed above knock limited speed against release of load on the brake load dynamometer.The torque variation with speed of engine is as shown in Fig. 5.2- a & Fig.5.2- b for various compression ratios with both fuels. The trend of the torque-speed variation is same for all compression ratios as it is the case for the base line tests (Fig. 5.1-a & Fig. 5.1- b). In these figures, torque is found to be reduced with increase in speed (Erkus et al.,2013)at a particular CR. However for a given speed, torque increases initially with increase in compression ratio and then decreases. This limiting CR is 5.02 for the engine speed of 2092 rpm in case of petrol. . If CR increases further to 5.27 then the secondary piston displacement is 4mm inside cylinder. This causes the spark location shifted to 4 mm inside combustion chamber. In this situation, when fuel gets ignited, the flame kernel developed takes more time to spread inside combustion chamber causing combustion delay and accordingly reduced power at output. That is the reason for the fact that the CR can not be further increased beyond 5.02 using novel VCR mechanism.
(a) (b)
Fig. 5.2: Variation of brake torque with modified head and VCR mechanism for petrol and kerosene
This observation hints for the presence of an optimum CR producing maximum torque at that speed. The maximum torque at CR 5.02 for gasoline is 18% higher than the torque at default CR of 4.8 (Fig.5.2-a). It may be worth noting that the increase in CR increases friction of the engine particularly between piston ring and cylinder wall of engine. So, there is a point at which further rise in compression ratio would not be profitable. This optimum CR is 5.27 for kerosene fuel as evident from Fig. 5.2(b). This observation is consistent with the fact that the lower heating value for kerosene is lower than that of gasoline which in turn hints for the higher optimum CR for kerosene.
(a) (b)
Fig. 5.3: Variation of brake power with modified head and VCR mechanism for petrol and kerosene.
The brake power variation with speed is shown in (Fig. 5.3-a & Fig.5.3-b) for various compression ratios starting from original CR of 4.8 to 5.4 for both fuels. As seen in figure, the brake power for a compression ratio increases initially up to certain speed and then decreases with further increase in speed for both the fuels. This trend has already been encountered in the baseline tests of these fuels. With the present VCR mechanism, at speed of 2124 rpm the brake power at CR 5.02 for petrol fuel is 12.08% higher than that of the brake power corresponding to CR 4.8 and same speed (Fig.5.3-b). With the increase in compression ratio, the cylinder pressure increases and in turn the brake mean effective pressure (BMEP) increases for the given valve opening and spark timing. The expected value of maximum
BMEP is corresponding to optimum condition of petrol is 0.69 bar. Thus, increased BMEP for a given swept volume enhances the efficiency of the engine. This justification is in well agreement with observation of literature reported findings from (Huang and Crookes,1998).Moreover, the power obtained for the compression ratios above the optimum CR of petrol is lower than the base value. The brake power with kerosene as fuel is found maximum at the CR of 5.27 and the percentage increment of 35.67% has been recorded over CR4.8 at speed 2095 rpm (Fig. 5.3-b). This observation is in line with the conclusion from the torque-speed plot for kerosene (Fig. 5.1-b). The reason for lower power with kerosene may be related to its density which is more than that of petrol. Therefore, higher compression ratio over optimum CR 5.27 adds more volume of fuel in to the cylinder which in turn increases the ignition time and results in the reduced power output (Dagaut and Michel,2006).
(a) (b)
Fig. 5.4: Variation of BTE with modified head and VCR mechanism for petrol and kerosene.
Thus, power output for kerosene gets reduced by 22% in the phase of further rise in compression ratio from 5.27 to 5.4.There is considerable change in brake thermal efficiency (BTE) with the compression ratio(Huang and Crookes,1998)and speed for petrol and kerosene (Fig. 5.4-a & Fig.5.4-b). For any choice of the fuel, BTE is found to be decreased with increase in speed at a particular speed condition. The maximum BTE for petrol is noted at CR 5.02 and it is 19% higher that the BTE of original CR of 4.8 (Fig.5.4-a). This is because of the fact that the increased compression ratio reduces the clearance volume and therefore increases the density of cylinder gases during burning. In turn, the peak pressure, temperatures are increased, thereby reducing the total combustion duration and ignition lag (Abdel and Osman,1997). Hence, the earlier observed optimum CR for gasoline retains its place from the analysis of efficiency as well. For kerosene also, the optimum CR from the perspective of efficiency is 5.27 from the Fig. 5.4-b. In this case, the maximum efficiency is noted to be 7.08% which is about 37% higher than the efficiency corresponding to normal compression ratio of 4.8. The brake specific fuel consumption (BSFC) is plotted for both the fuels (Fig. 5.5-a & Fig.5.5-b). The variation of BSFC with speed is same in either case and follows the same trend as that of the baseline test for a given compression ratio. It has been seen that the BSFC increases with increasing speed of the engine for the particular CR(Chandra et al., 2011).
(a) (b)
Fig. 5.5: Variation of BSFC with modified head and VCR mechanism for petrol and kerosene.
The heat loss to combustion chamber wall is proportionately greater and combustion efficiency is poor at high speed. Along with this, the friction power increases at a rapid rate with speed, resulting in slower increase in brake power which increases the fuel consumption (Abdel and Osman,1997). Apart from this fact, BSFC for a particular speed decreases with increase in compression ratio for petrol till CR equals to 5.02. At this CR, BSFC of petrol is found to be 17.64% lower with that of CR 4.8 (Fig.5.5-a). However, increase in CR, above the optimum one, tends to increase the BSFC substantially. Similarly for kerosene (Fig.5.5-b) also there is 37% decrement in BSFC when engine run at optimum CR 5.27 with that of CR 4.8. These inferences are aligned with the observations drawn from the efficiency plot. As shown in Fig.5.5a, the increasing trend is low upto 2150 rpm for CR4.9 as well CR5.02. But after 2150 rpm the BSFC shows increment with greater slope. The CR4.9 and CR5.02 could be achieved by 1 mm and 2 mm displacement of secondary piston inside combustion chamber respectively. With increase in CR, the cylinder pressure increases causing more homogeneous and complete combustion of charge. Therefore, BSFC for CR5.02 is found lower than that of CR4.9 over a speed range.