Posted by docsledge
Engine efficiency
Extracted from WikiPedia. Doesn’t include the info concerning diesel performance.
Engine efficiency of thermal engines is the relationship between the total energy contained in the fuel, and the amount of energy used to perform useful work. There are two classifications of thermal engines- (1) Internal combustion (gasoline, diesel and gas turbine, ie., Brayton cycle engines). (2) External combustion engines (steam piston, steam turbine, and the Stirling cycle engine). Each of these engines has thermal efficiency characteristics that are unique to it.
Modern gasoline engines have an average efficiency of about 25 to 30% when used to power an automobile. In other words, of the total heat energy of gasoline, 70 to 75% is rejected (as heat) in the exhaust or consumed by the motor (friction, air turbulence, heat through the cylinder walls or cylinder head, and work used to turn engine equipment and appliances such as water and oil pumps and electrical generator), and only about 25% of energy moves the vehicle. At idle the efficiency is zero since no usable work is being drawn from the engine. At slow speed (i.e. low power output) the efficiency is much lower than average, due to a larger percentage of the available heat being absorbed by the metal parts of the engine, instead of being used to perform useful work. Gasoline engines also suffer efficiency losses at low speeds from the high turbulence and head loss when the incoming air must fight its way around the nearly-closed throttle; diesel engines do not suffer this loss because the incoming air is not throttled. Engine efficiency improves considerably at open road speeds; it peaks in most applications at around 75% of rated engine power, which is also the range of greatest engine torque (e.g. in the 2007 Ford Focus, maximum torque of 133 foot-pounds is obtained at 4500 RPM, and maximum engine power of 136 brake horsepower (101 kW) is obtained at 6000 RPM).
The efficiency depends on several factors, one of which is the compression ratio. Most gasoline engines have a ratio of 10:1 (premium fuel) or 8:1 (regular fuel), with some high performance engines reaching a ratio of 12:1 with special fuels. The greater the ratio the more efficient is the machine. Higher ratio engines need gasoline with higher octane value, which inhibits the fuel's tendency to burn nearly instantaneously (known as detonation or knock) at high compression/high heat conditions.
It should be noted that at lower power outputs, the effective compression ratio is less than when the engine is operating at full power, due to the simple fact that the incoming fuel-air mixture is being restricted. Thus the effective engine efficiency will be less than when the engine is producing its maximum rated power. One solution to this fact is to shift the load in a multi-cylinder engine from some of the cylinders (by deactivating them) to the remaining cylinders so that they may operate under higher individual loads and with correspondingly higher effective compression ratios. This technique is known as variable displacement.
A gasoline motor 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 said to be stoichiometric, that is when burned, 100% of the fuel and the oxygen are consumed. Mixtures with slightly less fuel, called lean burn are more efficient, whilst slightly rich mixtures, with lower air fuel ratios produce more power at the expense of higher fuel consumption. The combustion is a reaction which uses the air's oxygen content to combine with the fuel, which is a mixture of several hydrocarbons, resulting in water vapor, carbon dioxide, and sometimes carbon monoxide and partially-burned hydrocarbons. In addition, at high temperatures the air's oxygen tends to combine with the air's nitrogen, forming oxides of nitrogen (usually referred to as NOx, since the number of oxygen atoms in the compound can vary, thus the "X" subscript). This mixture, along with the unused nitrogen and other trace atmospheric elements, is what we see in the exhaust.
I think the above pretty well covers the engine efficiency thing. Plus, it covers some of the AFR 14:1 EPA question.
Engine efficiency
Extracted from WikiPedia. Doesn’t include the info concerning diesel performance.
Engine efficiency of thermal engines is the relationship between the total energy contained in the fuel, and the amount of energy used to perform useful work. There are two classifications of thermal engines- (1) Internal combustion (gasoline, diesel and gas turbine, ie., Brayton cycle engines). (2) External combustion engines (steam piston, steam turbine, and the Stirling cycle engine). Each of these engines has thermal efficiency characteristics that are unique to it.
Modern gasoline engines have an average efficiency of about 25 to 30% when used to power an automobile. In other words, of the total heat energy of gasoline, 70 to 75% is rejected (as heat) in the exhaust or consumed by the motor (friction, air turbulence, heat through the cylinder walls or cylinder head, and work used to turn engine equipment and appliances such as water and oil pumps and electrical generator), and only about 25% of energy moves the vehicle. At idle the efficiency is zero since no usable work is being drawn from the engine. At slow speed (i.e. low power output) the efficiency is much lower than average, due to a larger percentage of the available heat being absorbed by the metal parts of the engine, instead of being used to perform useful work. Gasoline engines also suffer efficiency losses at low speeds from the high turbulence and head loss when the incoming air must fight its way around the nearly-closed throttle; diesel engines do not suffer this loss because the incoming air is not throttled. Engine efficiency improves considerably at open road speeds; it peaks in most applications at around 75% of rated engine power, which is also the range of greatest engine torque (e.g. in the 2007 Ford Focus, maximum torque of 133 foot-pounds is obtained at 4500 RPM, and maximum engine power of 136 brake horsepower (101 kW) is obtained at 6000 RPM).
The efficiency depends on several factors, one of which is the compression ratio. Most gasoline engines have a ratio of 10:1 (premium fuel) or 8:1 (regular fuel), with some high performance engines reaching a ratio of 12:1 with special fuels. The greater the ratio the more efficient is the machine. Higher ratio engines need gasoline with higher octane value, which inhibits the fuel's tendency to burn nearly instantaneously (known as detonation or knock) at high compression/high heat conditions.
It should be noted that at lower power outputs, the effective compression ratio is less than when the engine is operating at full power, due to the simple fact that the incoming fuel-air mixture is being restricted. Thus the effective engine efficiency will be less than when the engine is producing its maximum rated power. One solution to this fact is to shift the load in a multi-cylinder engine from some of the cylinders (by deactivating them) to the remaining cylinders so that they may operate under higher individual loads and with correspondingly higher effective compression ratios. This technique is known as variable displacement.
A gasoline motor 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 said to be stoichiometric, that is when burned, 100% of the fuel and the oxygen are consumed. Mixtures with slightly less fuel, called lean burn are more efficient, whilst slightly rich mixtures, with lower air fuel ratios produce more power at the expense of higher fuel consumption. The combustion is a reaction which uses the air's oxygen content to combine with the fuel, which is a mixture of several hydrocarbons, resulting in water vapor, carbon dioxide, and sometimes carbon monoxide and partially-burned hydrocarbons. In addition, at high temperatures the air's oxygen tends to combine with the air's nitrogen, forming oxides of nitrogen (usually referred to as NOx, since the number of oxygen atoms in the compound can vary, thus the "X" subscript). This mixture, along with the unused nitrogen and other trace atmospheric elements, is what we see in the exhaust.
I think the above pretty well covers the engine efficiency thing. Plus, it covers some of the AFR 14:1 EPA question.