Quote:
Originally Posted by omgwtfbyobbq
Engine efficiency, for everything from semi engines to small gasoline engines is a function of load and engine speed. High load and low engine speed are typically where efficiency is the best. Although, low load hurts efficiency more than high engine speed does IMLE. Efficiency is given in terms of grams of fuel used per kWh generated, so lower is better. Here are some examples. . .
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Thank you for the explanation. This is very instructive. One question: I understand that the vertical axis represents load on the engine, but is load the horsepower being demanded of the engine or just the torque? If I'm reading the maps correctly, I'm assuming that the vertical axis is only the torque component since horsepower is torque x rpm and rpm is represented separately on the horizontal axis. Am I correct?
So if a car needs say 20 hp to cruise down the highway at a given speed, then lowering the rpm (with taller gearing) increases the torque required in order to maintain the 20 hp output.
Now since the fuel consumption circles represent fuel quantity (grams, gallons, etc.) per hour of power output (hp, KW, etc.), then running the engine at a lower rpm that demands more torque (i.e., load), up to a certain point, to make that power gives the best efficiency.
So let's say again that you want to run at a speed that requires 20 hp. Since hp = torque x rpm x 1/5250, then at 2,000 rpm the engine would have to make 52.5 ft-lbs. If you run at 3,000 rpm, then the engine only has to make 35 ft-lbs. Now let's look at your fuel consumption map for the prius engine, which has the best high rev fuel efficiency of any of the engines you've shown. Making 20 hp at 2,000 rpm and 52.5 ft-lbs (71 Nm), fuel consumption is less than 240 g/kWh. At 3,000 rpm and 35 hp (47.5 Nm), fuel consumption goes up to 270 g/kWh. So even though the prius engine has very "flat" fuel consumption lines across the rpm range, as rpm increases, the engine load required to maintain a given horsepower drops, so engine efficiency drops. In other words, if you plot 20 hp on the map at different rpms, you get a line sloping down from left to right. So plot a given hp at various rpms on any of the fuel maps that you've provided and you find that the best efficiency comes in around 2,000 rpm or less.
Am I reading things correctly here?
) -- lesser-expensive wheels generally become heavier as they get larger, which translates into more energy given during acceleration unless momentum is conserved. I guess it depends on the type of driving...
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