Study finds hitting thermodynamic sweet spot in dilute, boosted gasoline engines has potential for fuel economy gains between 23% and 58%
A study by a team led by Dr. Dennis Assanis at the University of Michigan suggests that accessing the “thermodynamic sweet spot” in high-efficiency, dilute, boosted gasoline engines has the potential for vehicle fuel economy gains between 23% and 58%. Their paper is published in the International Journal of Engine Research.
Recent developments in ignition, boosting, and control systems have opened up new opportunities for highly dilute, high-pressure combustion regimes for gasoline engines. This study analytically explores the fundamental thermodynamics of operation in these regimes under realistic burn duration, heat loss, boosting, and friction constraints. The intent is to identify the benefits of this approach and the path to achieving optimum engine and vehicle-level fuel economy.—Lavoie et al.
Using a simple engine/turbocharger model in GT-Power to perform a parametric study exploring the conditions for best engine efficiency, they found:
Best engine efficiency in the mid-dilution range, a result of the tradeoff between fluid property benefits of lean mixtures and friction benefits of higher loads.
Dilution with exhaust gas is nearly as effective as air dilution when compared using a ‘fuel-to-charge’ equivalence ratio defined as Φ′ ≡ Φ (1-RGF) where RGF is the total residual gas fraction.
Optimal brake efficiencies are obtained over a range 0.45 ≤ Φ′ ≤ 0.65 for operation up to 3 bar manifold pressure, yielding peak temperatures under 2100 K and peak pressures under 150 bar.
These conditions are intermediate between homogeneous charge compression ignition and spark-ignition regimes and are, the team noted, the subject of much research.
George Lavoie, Elliott Ortiz-Soto, Aristotelis Babajimopoulos, Jason B. Martz and Dennis N. Assanis (2012) Thermodynamic sweet spot for high-efficiency, dilute, boosted gasoline engines. International Journal of Engine Research doi: 10.1177/1468087412455372