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engine power output

2246 Views 4 Replies 3 Participants Last post by  james
Hi there... I'm looking for information about a car's engine power output, such as the proportion of power used to accelerate car, to overcome friction, etc etc. Can anybody please help me? Thanks a lot...
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Interesting question. I can't answer it directly, but I can tell you that most of the power is used for acceleration - that's what makes the hybrid concept work.

If I look at my mpg gauge while accelerating on to a freeway, for instance, it will be down around 10-15 mpg, but when I'm cruising on the flat, it will be around 75. So a first guess would be that 70 mph, something under 20% of the Insight's full power (67 HP, IRRC) is used to overcome friction from drivetrain, rolling resistance from tires, and air resistance.
The question kind of intrigued me, so I figured out a way to get a sort of answer. There's a road I drive quite often, that drops about 2300 feet in 9 miles, at a fairly constant gradient. When the battery has fully charged, the Insight coasts at about 70 mph. This is in gear, so with fuel cutoff it ought to be getting all the engine friction, but no power.

Figure that with me and the dog in it, the Insight weighs about 2100 lbs. Multiply that by elevation, and you get total potential energy. Figure how long it takes to cover 9 miles at 70 mph, and you have power dissipation. I left the calculator at home, but if you crunch the numbers, you find that it takes about 17 HP to move the Insight at a steady 70 MPH on level ground.
Insight Efficiency

A gallon of gasoline has a higher heating value of about 140,000 Btu. 17 hp is equal to 43,255 Btu/hr, so if the Insight was 100 % efficient, 43,255 Btu/hr at 70 miles per hour would translate to 227 miles per gallon.

From this, and knowing your actual mpg at 70 mph on a level road, you can calculate the actual efficiency of your Insight at that speed.

If your average mileage at 70 mph is 70 mpg, the efficiency is 70/227 = 31 percent. This sounds about right.

Nothing quite as nice as doing the problem two ways, and getting the same answer.

I read that roughly 1/3 of the energy in the gasoline gets converted to power, another 1/3 goes out the exhaust, and the rest to the cooling system. And of the power part, some gets used in overcoming internal friction and such. Which is a good argument for the exhaust recovery turbine we were discussing a while back. Say you could recapture even 10% of the exhaust energy, and feed it back through the IMA. Since friction losses in the engine stay the same, you get well over 10% improvement in efficiency.

And if that means the engine spends more time in lean-burn mode... OK, who's got a good machine shop?

(I did a bit of a web search, and found that Catepillar is working on something similar, so I'm not totally off-the-wall :))
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