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A gasoline engine burns a mix of gasoline and air, consisting of a range of about twelve to eighteen parts (by weight) of air to one part of fuel (by weight). A mixture with a 14.7:1 air/fuel ratio is stoichiometric, that is when burned, 100% of the fuel and the oxygen are consumed.
In the case of a production gasoline engine, the most efficient BSFC is approximately 225 g/(kW⋅h), which is equivalent to a thermodynamic efficiency of 36%. An iso-BSFC map (fuel island plot) of a diesel engine is shown. The sweet spot at 206 BSFC has 40.6% efficiency. The x-axis is rpm; y-axis is BMEP in bar (bmep is proportional to torque)
Average fuel consumption values are 160–180 g/kWh for slower two-stroke diesel cargo ship engines using fuel oil, reaching up to 55% efficiency at 300 rpm; 195–210 g/kWh for turbodiesel passenger cars; 195–225 g/kWh for trucks; and 250–350 g/kWh for naturally aspirated Otto cycle gasoline passenger cars.
For a turbo diesel too low a gear will move the engine into a high-rpm, low-torque region in which the efficiency drops off rapidly, and thus best efficiency is achieved near the higher gear. [17] In a gasoline engine, efficiency typically drops off more rapidly than in a diesel because of throttling losses. [ 18 ]
The heart of the Mitsubishi's MVV system is the linear air–fuel ratio exhaust gas oxygen sensor. Compared with standard oxygen sensors, which essentially are on-off switches set to a single air/fuel ratio, the lean oxygen sensor is more of a measurement device covering the air/fuel ratio range from about 15:1 to 26:1. [19]
In 2005, BMW announced the development of the turbosteamer, a two-stage heat-recovery system similar to the Mack system that recovers 80% of the energy in the exhaust gas and raises the efficiency of an Otto engine by 15%. [10] By contrast, a six-stroke engine may reduce fuel consumption by as much as 40%.
Since HCCI engines are fuel-lean, they can operate at diesel-like compression ratios (>15), thus achieving 30% higher efficiencies than conventional SI gasoline engines. [2] Homogeneous mixing of fuel and air leads to cleaner combustion and lower emissions.
Fuel efficiency is dependent on many parameters of a vehicle, including its engine parameters, aerodynamic drag, weight, AC usage, fuel and rolling resistance. There have been advances in all areas of vehicle design in recent decades. Fuel efficiency of vehicles can also be improved by careful maintenance and driving habits. [3]