The effects of compression ratio on brake thermal efficiency at full load as well as part load operating conditions typical of those encountered in the CAFE test procedure are important considerations for maximizing light-duty vehicle fuel economy. Thermal efficiency of the ideal Otto cycle is given by this equation:
|Indicated Thermal Efficiency = 1 – 1/CR(1-k)||(1)|
Where: CR = compression ratio
k = ratio of constant pressure to con-
stant volume specific heats
Equation 1 shows that indicated thermal efficiency increases, but at a decreasing rate, as compression ratio is increased. However, mechanical efficiency decreases as compression ratio is increased due to higher loads on the pistons, rings, and bearings of the engine. Brake thermal efficiency is the product of indicated thermal efficiency and mechanical efficiency.
At full load conditions, mechanical efficiency can be relatively high. However, at part load conditions, mechanical efficiency will be significantly lower, even for relatively constant friction levels. The significant effect of load on mechanical efficiency is illustrated by the equation for mechanical efficiency:
|Mechanical Efficiency = BMEP/IMEP = BMEP/(BMEP + FMEP)||(2)|
Where: BMEP = brake mean effective pressure
IMEP = indicated mean effective
FMEP = friction mean effective
The effects of compression ratio on brake thermal efficiency together with indicated thermal efficiency and mechanical efficiency are shown in Figures 2.12(a) for full load conditions and Figure 2.12(b) for part load conditions. These figures provide the following insight into the effects of compression ratio on brake thermal efficiency:
- At full load, brake thermal efficiency increases, but at a decreasing rate, with increasing compression ratio, similar to indicated thermal efficiency.
- Up to 3 percent reduction in fuel consumption for naturally aspirated engines might be realized if compression ratio is increased from today’s typical level of 10:1 to approximately 12:1. Possibly greater reductions in fuel consumption might be realized for turbocharged engines capable of operating at higher boost pressures without knock so that further downsizing could be realized. Increasing gasoline octane from 91 RON of regular grade gasoline to 95 RON has been estimated to facilitate operation at a 12:1 compression ratio.
At part load, nearly insignificant improvements in brake thermal efficiency on the CAFE test cycles are expected to be obtained by increasing compression ratio beyond approximately 12:1 due to the increasingly lower mechanical efficiency.