Hitting 5.5k often.....

[Brucelee]

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.

[RandallNeighbour]

Jim, thanks for that good long explanation. I always learn a lot from you when you are in education mode

One follow up question. How often or when should I be adding an additive, or will running Chevron with Techron consistently keep the carbon deposits low?

[MNBoxster]

Originally Posted by RandallNeighbour

Jim, thanks for that good long explanation. I always learn a lot from you when you are in education mode

One follow up question. How often or when should I be adding an additive, or will running Chevron with Techron consistently keep the carbon deposits low?

Hi,

I add a Can of Jectron at the start of the season, in addition to working on engine deposits, it cleans the Injectors and Valves - the $10 Tune-Up '). Using good Gas and not storing it for extended period will usually be all you need to a year-round car...

Happy Motoring!... Jim'99

[porschegeorg]

Jim...

I thought I read somewhere on here that the injector cleaners may leave some stuff in the oil. Would it be better to run the cleaner 3000 miles prior to the next oil change? That way that stuff won't stay in the oil all year round?

Just wondering, since I have some techron I'd like to run in mine sometime. Just deciding when to use...

Thanks
Georg

[MNBoxster]

Originally Posted by porschegeorg

Jim...

I thought I read somewhere on here that the injector cleaners may leave some stuff in the oil. Would it be better to run the cleaner 3000 miles prior to the next oil change? That way that stuff won't stay in the oil all year round?

Just wondering, since I have some techron I'd like to run in mine sometime. Just deciding when to use...

Thanks
Georg

Hi,

It is better to do this prior to an Oil Change, but in the case of the Jectron, not absolutely necessary. Any Jectron which get into the Oil will Flash off. So far as Techron, I cannot say. Check their MSDS sheet and look at the volatility statistics, that should tell you...

Happy Motoring!... Jim'99