Are gas engines ever going to catch up?
Our creaky old Cummins powered truck (bought used) seats 4, does about 20-23mpg bobtail and 12mpg towing a 5th wheel trailer at 22,999lbs. It's mileage is somewhere in between hauling a camper and/or...
Forum Discussion
There is one crucial component to the diesel vs. gas equation that rules the day: compression ratio. And this is because compression ratio determines where you start calculating everything else. This is because the compression ratio determines theoretical thermal efficiency, and as we all know, the theoretical, achievable-only-in-a-perfect-world state is the best you can do.
And it works like this: very simply theoretical thermal efficiency is a function of compression ratio. The higher the compression ratio, the higher the thermal efficiency of a piston engine. There is, to be sure, a law of diminishing returns here, in the sense that increasing the compression ratio from 8.5:1 to 9.5:1 gets you more of a bump than going from 12.5:1 to 13.5:1.
Gas engines today use compression ratios from 8.5:1 upward to around 12:1, the latter requiring racing high-octane gas. Modern diesels run 17.5:1 to around 22:1, although due to thermal losses concerned with the shape of the combustion chamber, 16.5:1 or 17:1 is believed to be optimal.
So as a result, gas engines currently get around 32% efficiency, diesels more like 43%. And until "they" learn to get that compression ratio up to around 16 or 17 to one, gas engines are simply not capable of the kinds of power and efficiency you'll get from diesels.
However, as others have noted, the latest crop of diesel engines are even better and quieter than your old Cummins truck. If you want an education of what is possible, pop over to your nearest BMW or Mercedes stores and as to see one of their diesel cars. I checked out 2011 X5 BMW diesel a year or so ago on the used car lot at the BMW store. Sitting in it while it was running, walking around it as it idled, you couldn't hear the engine, it was so quiet. That engine is a 3.0 liter 6 cylinder power plant that puts out over 400 ft lbs of torque.
And it works like this: very simply theoretical thermal efficiency is a function of compression ratio. The higher the compression ratio, the higher the thermal efficiency of a piston engine. There is, to be sure, a law of diminishing returns here, in the sense that increasing the compression ratio from 8.5:1 to 9.5:1 gets you more of a bump than going from 12.5:1 to 13.5:1.
Gas engines today use compression ratios from 8.5:1 upward to around 12:1, the latter requiring racing high-octane gas. Modern diesels run 17.5:1 to around 22:1, although due to thermal losses concerned with the shape of the combustion chamber, 16.5:1 or 17:1 is believed to be optimal.
So as a result, gas engines currently get around 32% efficiency, diesels more like 43%. And until "they" learn to get that compression ratio up to around 16 or 17 to one, gas engines are simply not capable of the kinds of power and efficiency you'll get from diesels.
However, as others have noted, the latest crop of diesel engines are even better and quieter than your old Cummins truck. If you want an education of what is possible, pop over to your nearest BMW or Mercedes stores and as to see one of their diesel cars. I checked out 2011 X5 BMW diesel a year or so ago on the used car lot at the BMW store. Sitting in it while it was running, walking around it as it idled, you couldn't hear the engine, it was so quiet. That engine is a 3.0 liter 6 cylinder power plant that puts out over 400 ft lbs of torque.
