Let's Talk Diesel

More than once someone said "I won't have a diesel because diesel fuel costs more than gasoline". I think; geez, I won't comment to this dude on that. But, then I wondered just how much more economical is a diesel? You see on TV how train locomotives haul one ton of freight 400-some miles on a gallon of diesel. And those big rigs on the road weighing 80,000 pounds getting 4 MPG. Could you imagine if those trucks were burning gasoline? one or two MPG?? Or, maybe how many gallons per mile?
Let's do some really fun math......
We'll compare a 3000 pound sedan running on gas and getting 36 MPG on the highway against the previous generation WV Jetta TDI diesel, also weighing in at 3000 pounds. Fair enough?
The Jetta gets over 50 MPG on the highway. In fact, we measured 54 MPG on a trip to Boston and back. Another trip we documented was at 52 MPG.
As of today the national average price of gas and diesel was $2.71 and $2.90 per gallon respectively. Gasoline has a heat value of 114,100 BTU/gal and diesel 130,500 BTU/gal. That's 14% more energy at only 7% more cost.
But there's more. A modern gasoline engine is 30% efficient, while a diesel is 40% efficient. Both efficiencies are rated at highway speeds in overdrive or 5th gear. Now, if we take those BTU/gal values and multiply out at .3 and .4 we get 34,230 BTU gal on gas and 52,200 BTU/gal for diesel. This is the amount of heat turned into work by the engines. The rest of the heat goes out the cooling and exhaust systems. The diesel delivers 50% more work than the gas engine.
Let's look at the MPG figures again, 36 for gas, 54 for diesel. Amazing, 50% better mileage.
What does that mean economically? For every gallon we burn in the diesel we go 18 more miles down the road than on gas. This equates to $0.95 saved per gallon. We paid $8000 for our used Jetta, can you figure out how many highway miles we would have to travel to pay for this vehicle?
Of course, there is the issue of pollution. A diesel is dirtier than a gas engine. (except the lastest generation of clean diesels) But, the pollutant outputs are measured per mile driven. More mileage, less pollutants per mile. If you add only 2-5% biodiesel, you sweeten the exhaust and is the best anti-wear additive there is. If this prevents your engine from wearing out sooner, just how green is it to prevent building another engine to replace it?
Americans don't like diesels like Asian and European countries. That why the selection is quite limited in the states. Most diesels are found in 3/4 ton and up pickups, few sedans.
When we drive down the highway, I can go a hundred miles before seeing another car that matches our mileage. Feels good, I get a little smug. Even a Prius can't catch us, except in the city, where it benefits from regeneration of electricity from braking.