Increasing compression ratio (CR) is one of the most fundamental ways to improve engine efficiency, but the CR of practical spark ignition engines is limited by knock and spark retard at high loads. A variable CR mechanism could improve efficiency by using higher CR at low loads, and lower CR (with less spark retard) at high loads.
This paper quantifies the potential efficiency benefits of applying variable CR to a modern downsized, boosted gasoline engine. Load sweeps were conducted experimentally on a multi-cylinder gasoline turbocharged direct injection (GTDI) engine at several CRs.
Experimental results were compared to efficiency versus CR correlations from the literature and were used to estimate the fuel economy benefits of 2-step and continuously variable CR concepts on several engine/vehicle combinations, for various drive cycles.
For vehicles with automatic transmissions, the fuel economy benefit of 2-step variable CR was estimated to be 2.5-3.1% on the EPA Metro-Highway cycle, and 0.8-1.2% on the US06 cycle relative to a fixed 10:1 CR baseline. The benefit of continuously variable CR for the same vehicles was estimated to be 2.7-3.3% EPA Metro-Highway, and 1.7-2.1% US06. The effects of CR on exhaust gas temperature, peak cylinder pressure, burn rate, low-speed torque, and high-speed power were also examined.
