Thermodynamic sweet spot for high-efficiency, dilute, boosted gasoline engines

Abstract

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. A simple engine/turbocharger model in GT-Power is used to perform a parametric study exploring the conditions for best engine efficiency. These conditions are found 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 F 0 [ F (1-RGF) where RGF is the total residual gas fraction. Optimal brake efficiencies are obtained over a range 0.45 4 F 0 4 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 subject of much current research on advanced combustion modes. An engine-vehicle drive train simulation shows that accessing this thermodynamic sweet spot has the potential for vehicle fuel economy gains between 23% and 58%.

Mixture Dilution on a Natural Gas SI Engine Operating at Low Load

“…This is especially valid for air dilution. The measured positive effect of the mixture dilution is in accordance with the theoretical study presented by Lavoie et al [1]. Experimental data shows a less significant effect in the case of EGR, which is probably caused by chemical efficiency as shown in Figure 3.…”

“…The VTG actuator was kept permanently in a position for maximum stator blades crosssectional area (= minimum boost pressure, which corresponds to the typical adjustment at low engine load). The starting point for each measuring sequence was adjusted to the above-mentioned speed and load, l = 1, and angle position of normalized heat release value of 50% = 10° crank angle after compression top dead center (CA50 = 10°CA aTDC) in accordance with the optimum combustion phasing mentioned in [1]. The closed loop l-control was running and was responsible for mixture strength adjustment at the starting point.…”

“…Experimental data shows a less significant effect in the case of EGR, which is probably caused by chemical efficiency as shown in Figure 3. The theoretical study [1] did not take the chemical efficiency into account and therefore shows approximately equal significance of both means of dilution. The raw exhaust gas composition is the next relevant output.…”

“…If lean mixture is implemented, the achievable power needs to be compensated by increased boost pressure. Low burning velocity of the lean mixture additionally reduces NO X formation, but it also worsens engine efficiency, especially if not compensated by optimized combustion phasing [1]. Poor flammability of the lean mixture causes high cycle-to-cycle variability.…”

Grand Challenges in Engine and Automotive Engineering

“…A starting point for discussion of engine efficiency is the theoretical air-standard Otto cycle efficiency, which indicates that engine efficiency increases with increased compression ratio. [Other parameters that also in uence efficiency are described by Lavoie et al (2012)]. Early spark ignition (SI) engines were plagued by "spark knock, " which created explosive pressures that would damage engines and limited the compression ratio to about 4:1.…”