Engine thermal efficiency

When talking about fuel mileage, there are two "umbrella" factors to work with:

1) Lowering the horsepower requirement of the engine

We do this by slowing down, putting more efficient tires on, driving more aerodynamic trucks, etc. It's quite simple the faster you go, the more horsepower you need. Horsepower equals fuel, that moves us on to number two:

2) Using the least amount of fuel for a certain amount of horsepower

This is more voodoo black art stuff, well not quite. If a certain setup at a certain speed used 250 hp, and that was the absolute minimum power we needed to maintain speed, and there was nothing else to reduce this, how do we go about increasing fuel mileage? We look at the engine.

The first and most obvious thing to do is choose the correct gearing. But even this can get vague. We talk about "sweet spots" all the time, but what does this really mean? Who decided the sweet spot, and is it actually the most efficient rpm or is it a compromise? Let me introduce a concept to you called BSFC or "brake specific fuel consumption". There are actually two formulas regarding thermal efficiency of engines. The first is a percentage. It is described as:

Thermal efficiency = output of engine / total BTU's available in the fuel

It's quite simple, how much of the fuel are we converting to motive power? If there's 100 units of heat in the fuel, and we convert 40 to power, we're operating at 40% efficiency, which is around what our diesels run at. If we hit 100% (impossible) we'd have no need for cooling systems and our EGTs would be ambient temperatures.

The second formula is actually the same thing, only with different numbers. With this formula, lower is better. It is described as how many lbs of fuel does it take to make one horsepower for one hour. The formula looks like this:

BSFC = lbs of fuel/ hp - hr

Here's a chart for a series 60:

So we can see that the engineers made a good choice, 1500 rpm seems good. But notice how above 1800 rpms and below 1300 efficiency drops at our horsepower levels. But why not 1450 rpms? It would be even more efficient. It's probably due to driver satisfaction. The sad thing is BSFC charts are no longer available. They're top secret. Trade secrets if you will. Don't want the dumb truck drivers making their own decisions.

One thing you will notice is only at peak horsepower is the engine most efficient. This seems counterintuitive, and it sort of is. That's because when we're at peak horsepower we're also getting the worst fuel mileage. That's because we're either wasting horsepower by driving fast, or storing it as momentum going up a hill. That's why instant readouts only matter on flat ground at a steady speed.

If we had access to BSFC charts, I could tell you which of the new engines is the most efficient. It's not hard to read a BSFC chart. But the good news is with new "fuel efficiency laws" we may now have access to the charts. Or at least I hope we do.

So when we modifiy our engines for fuel mileage, what we're doing is making the same horsepower with less fuel, lowering the BSFC numbers making them more thermally efficient. One thing I've found is speed is far more important than sweet spot. Runnig slower out of the sweet spot will always get better MPG than running faster in the sweetspot. Notice on the chart at 240 hp between 1200-1800 rpm we're in the same efficiency island. That means out of the sweet spot might cost us 1% in fuel mileage at most, but that's only at 240 hp. But slowing .5 mph will do more than 1%.

Well enough of this rambling.

 

Yes it's my inner nerd bursting out at the seams. I'm very much a science/math person. I obsess over the little things. I calculate my fuel mileage every tank. I can usually guess within 2 tenths before I fill up.