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Discussion Title:  best RPM range for normal driving

pw981

2004-08-25 10:45:00

best RPM range for normal driving

my vechicle has a Renault 1.2 16v petrolengine, max 75hp @ 5500RPM and max torque is @ 3500RPM. I'm just wondering what is the best engine speed for normal/slow driving conditions in order to save fuel? I live in the UK and with really high fuel prices driving style is a major contributing factor! I usually change gear just over 2000 RPM, if i'm on a long straight slow road I just put it in 5th and it is doing 1,500 RPM at 30MPH. The engine sounds normal at this speed as in no labouring. My Renault dealer says driving the car with engine speeds this low is putting a lot of strain on it. Its got me confused, can anyone help please?



Rufe

2004-08-25 18:21:00

I am not familiar with the Renault motor, so take this reply with a grain of salt. I think you should shift the transmission at a higher rpm, however once at a stable speed, at 30 MPH, I would be in 4th gear, not 5th.

Do you have an owner's manual? What is the recommended shift range there? Is there a magazine test with your motor that shows the power curves? Where do they start measuring?

Gas mileage has a lot to do with starting and stopping, and of course the accellerating after the stop is what kills your total mileage. Once up to highway speeds, air drag becomes an issue. You need more horsepower to overcome the increased drag, and more gas to accelerate to that speed. So, you are on the right track, but perhaps a bit too extreme.

I do not run any of my engines below 2000 RPM's for any extended period of time, and tend to agree with the gentleman from the Renault Dealership.



SaabJohan

2004-08-26 08:10:00

Re: best RPM range for normal driving

Probably the Renault dealer doesn't know what he's talking about. For the lowest fuel consumption the engine should be driven with the highest load as the main reason for the gasoline engines inefficiency is caused by throttling, this means that the engine should be driven with the highest gear and lowest engine speed possibly at constant speed. At really low speeds the engine can start to vibrate, this is no danger for the engine but it will be a little harder on the engine mounts which must take the vibrations, it's also not that fun to ride in a car that vibrates heavily.

When you accelerate this should be done with almost full throttle, so you get your speed as fast as possible. Also use a low gear.

Then you must also plan your driving, don't stop if possible, don't drive too slow or too fast and soo on.

When an engine runs most efficient it converts over 30% of the energy in the fuel, this is done with almost full throttle at a engine speed close to peak torque, the engine speed is however less important. When you use part throttle the efficiency drops, it can drop as low as 10-15%. 30% efficiency will be equal to around 250 g fuel / kWh while 10% is around 800 g / kWh or so. If you drive 70 km/h a normal car will need approx. 7kW, with a speed of 70 km/h that means that with 30% efficiency your car consumes 3.4 litres/100 km while 10% needs 11 litres/100 km.



pw981

2004-09-04 11:37:00

thanks! anyone got any more opinions?



TRD2000

2004-09-08 20:52:00

Re: best RPM range for normal driving

yeah i'd say try and stay around that peak torque thing... but hell i dunno... i just try to stay below my turbo if i'm saving fuel...

shift point and where you drive depends on hills id say its flat near you... idling in top gear can't be bad... if you're not accellerating you dont need much from your engine... just don't labour it...

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SaabJohan

2004-09-10 09:53:00

Re: Re: best RPM range for normal driving

Quote:
Originally Posted by TRD2000
yeah i'd say try and stay around that peak torque thing... but hell i dunno... i just try to stay below my turbo if i'm saving fuel...

shift point and where you drive depends on hills id say its flat near you... idling in top gear can't be bad... if you're not accellerating you dont need much from your engine... just don't labour it...

Staying around torque peak isn't good, keep the rpm as low as possible. If the turbocharger can deliver boost at that rpm doesn't matter at all.

Acceleration should be done fast, so no low rpm acclerations.



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