Quote:
Originally Posted by Tesla
So what you are saying is basically the same as I wrote a couple of posts up, although mine is a Diesel, this is what I have generally done with most vehicles I drive.
I have a question though, how does this relate to the concept of using a vacuum gauge on gas engines as an economy gauge, i.e. higher vacuum, better economy?
Surely running in a higher gear with low engine speed and a more open throttle would result in lower vacuum & hence lower economy, only one concept can be right.
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In a throttled IC engine a higher gear and lower RPM-higher load will give you better mileage at steady speeds.
To better understand load, think of driving with the throttle locked in a position, say 25%. You start climbing a hill that gets steeper the more you climb. Now you are using fuel to maintain your speed as well as increase your altitude. In essence you are lifting the car higher using the hill as an inclined plane.
As the hill's grade increases your car has to work harder to maintain it's speed, and you have to increase the throttle position to keep it going at the same speed. This is due to increased load from lifting the weight of your car and it can be calculated precisely using the formula for horsepower.
At some point as the grade keeps increasing you can no longer maintain the same speed in the highest gear. This is the maximum load the engine can produce at that RPM.
In order to maintain the speed you will now have to downshift to a lower gear and increase your RPM to maintain the same speed. As the percentage of the grade increases you will have to again increase the throttle position until it is at maximum, then downshift to an even lower gear to maintain the speed.
Load is not directly related to throttle position, but in many cases you will have to increase the throttle position to maintain speed uphill because the work required to climb the grade is added to the work required to maintain a constant speed. In a way a hill allows you to store energy in the mass of your vehicle which you can use on the downhill portion of your trip, in some cases you can even cut the engine off and maintain the same speed without any fuel being consumed.
Driving With Load means you are trying to stay at the same vacuum reading regardless of the changes in elevation. It's a good strategy in hills as long as they are not too steep. Another strategy is to increase your speed on the downhill portion and let it drop off on the uphill portion (like an unpowered roller coaster).
When your engine vacuum is high, efficiency is low, even though your mileage may be it's best. This is because high vacuum means low compression, which means less work for the same amount of fuel consumed.
A 4 cylinder engine producing 20 HP at 2000 RPM uses X amount of fuel. If you increase the load so the same engine is running at the same speed but producing 50 HP, the fuel consumption will be 1.5X. Basically the extra 30 HP only cost you half again as much fuel.
At the 20 HP load you are using .5X fuel per 10 HP.
At the 50 HP load you are using .3X fuel per 10 HP.
This is why pulse and glide works well to increase your mileage. It's almost the same as climbing a hill then coasting down the other side, except there is no hill. You are using the mass of the vehicle to store inertia and then using the inertia to travel with no fuel consumption (assuming you shut the engine off). Even if you let the engine idle, coasting at 60 MPH with an engine using .25 GPH fuel, you are getting 240 MPG at 60 MPH coasting.
regards
Mech