Engine Efficiency and Gear Ratio

Pumping Losses, Operating Points & Drive Cycle Optimization

Counter-intuitively, operating an engine at wider throttle openings and lower RPM (higher gear ratios) drastically reduces throttling vacuum losses and friction, achieving up to 30% lower brake specific fuel consumption (BSFC).

We all know that fuel consumption is related to throttle position: more throttle equals more fuel consumption, right? Actually, it is a little more complicated—and rather counter-intuitive. In fact, under many operating conditions, opening the throttle wider will actually result in lower fuel consumption for a given power output.

Consider an engine producing a constant power output: running at 3,000 RPM at 100 Nm of torque produces the exact same power as running at 6,000 RPM at 50 Nm of torque. However, these two operating states exhibit dramatically different fuel consumption rates. At 3,000 RPM / 100 Nm, the engine operates near full throttle where air enters the cylinder with minimal restriction. At 6,000 RPM / 50 Nm, the throttle valve is partially closed, forcing the engine to expend energy sucking air past the intake restriction—a loss known as throttling pumping loss. Combined with higher mechanical friction at elevated RPM, fuel consumption can increase by up to 30% in lower gears for identical vehicle cruise speeds.

In dynamometer laboratory drive-cycle testing on motorcycles, smaller displacement engines (70cc–90cc) operated near full throttle proved up to 25% more fuel-efficient than larger 120cc–135cc engines over identical urban drive cycles. The smaller engines operated at wider throttle settings, eliminating throttling vacuum losses while benefiting from reduced piston ring friction.

Dynamometer testing also revealed that proper engine tuning directly affects efficiency: 5% to 10% of in-use motorcycles suffered from worn fueling components or poor calibration, consuming excessive fuel and outputting high hydrocarbons and carbon monoxide. Simple component replacement restored original fuel economy (up to 54–60 km/liter for 4-stroke models).