A Second Law Analysis of High Efficiency Low Emission Gasoline Engine Concepts

A second law analysis of spark ignition (SI) engines has been conducted to identify areas in which work capability is presently lost due to either thermodynamic irreversibilities or undesirable work transfers. The impact of advanced combustion strategies on raising powertrain efficiency is assessed. The modeling study relies on two simulation codes: a one-dimensional gas-dynamic simulation code for air flow and heat transfer external to the cylinder, and a single-cylinder thermodynamic cycle simulation code modified to incorporate a second law (Availability) analysis. Modeling results are presented for a base case 3.0 liter, port fuel injection (PFI) gasoline engine with a 10.5 compression ratio (CR), operated homogeneously with a stoichiometric fuel/air ratio. The effects on engine efficiency of lean burn operation, increased CR, and homogeneous charge compression ignition (HCCI) are assessed via two additional cases: a) a lean burn (λmax=1.7), direct injection, 12 CR, 2.9 liter engine; and b) an ultra-lean burn (λmax=5.0) direct injection, 16 CR, 2.7 liter engine. These strategies show the potential to raise the peak engine thermal efficiency from ∼35% to 43%. The Availability analyses show that while lean burn operation leads to higher engine efficiency due to reduced exhaust and in-cylinder heat losses, combustion irreversibilities are increased (as a fraction of available fuel energy), reflecting the lower temperature at which heat release occurs. The combustion losses are greatest for the ultra-lean burn case, as it operates at the highest λ values. The effects on work capability losses of HCCI vs. SI operation are estimated by considering the differences in burn rate, heat transfer, and combustion phasing. The analyses indicate that HCCI offers modest efficiency improvements vs. an optimized lean burn SI engine, though important indirect benefits exist through reduced Nox production. Fuel implications for reducing second law losses are discussed, and concepts for reducing the remaining losses are proposed.