With the new 'clean diesels' coming in greater numbers, I thought it would be a good time to discuss diesels, and throw in a few comparisons to hybrids and see what happens.
Pros of Diesels
- Fuel Economy
- Lower emissions (CO2)
- Higher Torque / More Power
- Engines last a long time
The biggest pro when it comes to diesels is the efficiency of the oil burner. The engine is actually simpler (as long as we don't worry about the emissions, and more power can be derived from a given gallon of fuel. That means we can get higher torque and better fuel efficiency out of a diesel than we can get in a gasoline engine.
And the higher fuel efficiency means you get lower carbon dioxide emissions, which you would think would mean environmentalists and lawmakers would jump at the chance to promote diesels. But it's not that clear cut when it comes to emissions, but I'll talk more about that later.
In fact, diesels can be more efficient than hybrids on the highways, which is why you see some recommend diesels over hybrids when you have long highway commutes. I've seen guidelines range from 25-40% of fuel economy gains over conventional gas engines. Or if you plan on towing or loading heavy loads, you are more likely to want a diesel engine. That's why diesel-hybrids are the choice of the rail roads.
Also, the engines tend to be sturdier. Just look at the recommendation for boating engines and you'll see that they can last a long, long time.
Cons of Diesels
- Emissions (Soot)
- Oil to Gallon
- Sound (not so bad anymore)
- Cost of Engine
- Cost of Fuel
- Cost of Clean/Filters
The cons when it comes to diesels look an awful lot like the pros sometimes.
First, the emissions. The lower carbon dioxide emissions is a good thing, but it's not the only thing. Unfortunately, oil burners tend to emit more soot. Soot can cause all sorts of issues for human beings and emitting more soot is something to be avoided.
Clean diesels are much better than diesels about emissions, and in order to be sold in the US, diesels have to meet stricter guidelines as to their emissions than they used to. Most of that comes from the filters they install in the engines.
The second con is how much oil is actually used to create a gallon of diesel vs. a gallon of gasoline. Despite higher fuel efficiency, since the amount of oil used to create a gallon of diesel is higher, the advantage from a national perspective isn't as much as you would hope for. Importing oil into the US is a costly and dangerous business. Diesel beats out gasoline here, but not by as much as most people think when all they look at is the fuel efficiency of the respective vehicles.
The third con is sound. But that's not so much of a con anymore. No, diesels aren't as quiet as gasoline engines, but the difference isn't really noticeable (unlike 20-30 years ago). Of course, you can't beat out the hybrid, which turns off the gasoline engine when you come to a stop. But still, sound isn't much, if any, of a con any more.
Also, diesel fuel is not available at every gas station. You would have to search for the locations, and you wouldn't be guaranteed an easy find if you took it on a longer trip. This isn't much of a con, because you would just have to be a little more careful about filling up.
The fourth con, which may be the worst one, is cost. Diesel engines cost more to build and install (those filtering systems aren't cheap). Diesel actually costs more than gasoline right now (it's pretty even, but still the edge goes to gasoline). That frays some of the cost savings you would expect from higher fuel efficiency. And, those filtering systems add to the cost to maintain. The urea systems need to be refilled at regular intervals.
Why Not Hybrid Diesels?
Good question, if you were wondering that. The problem with diesel hybrids comes down to cost again. Let's say a hybrid engine cost $X thousand more than a conventional car, then the diesel engine costs $Y more than the gasoline engine. Then a diesel hybrid costs $X+Y more than the conventional car.
You do get higher torque, better fuel economy, and a lot of other bonuses. But selling US buyers on a hybrid electric was probably hard enough. Selling them on a higher cost to buy a diesel-electric would have been extremely hard to do.
But plug-in hybrid electric vehicles (PHEV) or extended range electric vehicles (E-REV) are a much better fit for diesel engines. The higher fuel efficiency, coupled with the likelihood of long ranges of all electric power, could be a fuel efficiency maniac dream. And the reasons for making diesel-electric engines for trains come into play when you look at E-REVs.
Add in the fact that plug-ins are going to be expensive in the first place, and the added cost of a diesel (vs a gas) engine mated to the electric motor, and the added expense isn't really all that much (percentage wise).
Diesels, like everything it seems, just aren't clear cut winners. The clean diesels are a much better sell, but the cost differential may be hard to overcome. If diesel (the fuel) prices were to come down a little, the difference in fuel efficiency would be become readily apparent to most consumers. Without it, however, and the average consumer is going to stick with gasoline.