Abstract
The effects of variable compression ratio (CR) and fuel composition on thermal efficiency were investigated in a homogeneous charge compression ignition (HCCI) engine using blends of n-heptane and toluene with research octane numbers (RON) of 0 to 88. Experiments were conducted by performing CR sweeps at multiple intake temperatures using both unthrottled operation, and constant equivalence ratio conditions by throttling to compensate for varying air density. It was found that CR is effective at changing and controlling HCCI combustion phasing midpoint, denoted here as CA 50. Thermal efficiency was a strong function of CA 50, with overly advanced CA 50 leading to efficiency decreases. Increases in CR at a constant CA 50 for a given fuel composition did, in most cases, increase efficiency, but the relationship was weaker than the dependence of efficiency on CA 50. The increase in efficiency with higher CR was fuel-dependent, so that the fuels requiring a higher CR to achieve ignition did not gain a proportionate efficiency increase. For example, n-heptane achieved an indicated thermal efficiency (ITE) of 38% at a CR of 9:1, whereas a 50 wt% blend of toluene with n-heptane required a CR of 12:1 to achieve the same ITE. A simple heat balance around the engine showed that higher toluene content fuels had higher cooling losses. The high toluene fuels exhibited higher rates of maximum pressure rise than the lower octane fuels. The increased cooling losses can be attributed to the higher pressure rise rates, which are a driving force for heat transfer.
