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Brucelee · 03-25-2007, 08:04 AM

Interesting. See the bolded comments on higher RPM impacts on engine internals.

Date: Wed Jun 7 11:55:45 2000
Posted By: David Ellis, Researcher, NASA Lewis Research Center
Area of science: Engineering
ID: 959212290.Eg
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Message:

I am assuming you are referring to overall fuel efficiency of the car in your question.

Fuel efficiency is generally referred to as miles per gallon of gasoline burned or MPG. The higher the number, the higher the efficiency of the engine. For a given engine, two main things determine the fuel efficiency: the gear ratio for the transmission and the amount of fuel consumed by the engine.

The transmission ratio is critical to efficiency because the ratio determines how many times the wheels turn per engine cycle. In a low gear, i.e., first gear, the wheels turn less than once per time the drive shaft turns. This produces the high torque needed to get a car moving, but it would be very inefficient at high speeds.

Higher gears have the drive shaft turn fewer times until in either third or fourth gear the drive shaft turns about once per revolution of the wheel. In modern cars equipped with overdrive, the highest gear, normally fourth or fifth gear, will have the wheels turn more than once per revolution of the drive shaft. Very little torque is produced relative to first gear, but the ratio means you get the most forward progress out of each cycle of the engine. This produces the highest efficiency for a given engine speed or revolutions per minute (RPM). Since you have to be going around 35 mph (55 kph) to get a car into overdrive, the first part of the answer is yes, the efficiency of a car engine increases with increasing speed.

However, what happens once you start to increase your speed further? Increasing speed means increasing the RPM of the car engine. As the engine speeds up, the mechanical efficiency of the engine parts tends to decrease due to increased heat, friction, vibration, noise, etc.

The exact speed where the maximum occurs is specific to each engine and can change with time as parts wear. Some increases in efficiency may occur as the speed increases because increased temperature removes the built-in allowances for thermal expansion and allow such items as the piston rings to seal for maximum efficiency. In general, though, the maximum efficiency is realized around the speed at which the car shifts into its highest gear.

03-25-2007, 08:04 AM	#37
Brucelee Registered User Join Date: Jun 2004 Location: Des Moines, IA Posts: 8,083	Interesting. See the bolded comments on higher RPM impacts on engine internals. Date: Wed Jun 7 11:55:45 2000 Posted By: David Ellis, Researcher, NASA Lewis Research Center Area of science: Engineering ID: 959212290.Eg -------------------------------------------------------------------------------- Message: I am assuming you are referring to overall fuel efficiency of the car in your question. Fuel efficiency is generally referred to as miles per gallon of gasoline burned or MPG. The higher the number, the higher the efficiency of the engine. For a given engine, two main things determine the fuel efficiency: the gear ratio for the transmission and the amount of fuel consumed by the engine. The transmission ratio is critical to efficiency because the ratio determines how many times the wheels turn per engine cycle. In a low gear, i.e., first gear, the wheels turn less than once per time the drive shaft turns. This produces the high torque needed to get a car moving, but it would be very inefficient at high speeds. Higher gears have the drive shaft turn fewer times until in either third or fourth gear the drive shaft turns about once per revolution of the wheel. In modern cars equipped with overdrive, the highest gear, normally fourth or fifth gear, will have the wheels turn more than once per revolution of the drive shaft. Very little torque is produced relative to first gear, but the ratio means you get the most forward progress out of each cycle of the engine. This produces the highest efficiency for a given engine speed or revolutions per minute (RPM). Since you have to be going around 35 mph (55 kph) to get a car into overdrive, the first part of the answer is yes, the efficiency of a car engine increases with increasing speed. However, what happens once you start to increase your speed further? Increasing speed means increasing the RPM of the car engine. As the engine speeds up, the mechanical efficiency of the engine parts tends to decrease due to increased heat, friction, vibration, noise, etc. The exact speed where the maximum occurs is specific to each engine and can change with time as parts wear. Some increases in efficiency may occur as the speed increases because increased temperature removes the built-in allowances for thermal expansion and allow such items as the piston rings to seal for maximum efficiency. In general, though, the maximum efficiency is realized around the speed at which the car shifts into its highest gear. __________________ Rich Belloff