<P> Modern engines are often intentionally built to be slightly less efficient than they could otherwise be . This is necessary for emission controls such as exhaust gas recirculation and catalytic converters that reduce smog and other atmospheric pollutants . Reductions in efficiency may be counteracted with an engine control unit using lean burn techniques . </P> <P> In the United States, the Corporate Average Fuel Economy mandates that vehicles must achieve an average of 34.9 mpg (6.7 L / 100 km; 41.9 mpg) compared to the current standard of 25 mpg (9.4 L / 100 km; 30.0 mpg). As automakers look to meet these standards by 2016, new ways of engineering the traditional internal combustion engine (ICE) have to be considered . Some potential solutions to increase fuel efficiency to meet new mandates include firing after the piston is farthest from the crankshaft, known as top dead centre, and applying the Miller cycle . Together, this redesign could significantly reduce fuel consumption and NO emissions . </P> <P> Starting position, intake stroke, and compression stroke . Ignition of fuel, power stroke, and exhaust stroke . </P>

Working of four stroke petrol engine with neat sketch