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Diesel: Why does it get better mileage?
With every gas price hike, the popularity of diesel vehicles gains momentum. It’s no surprise, given their better fuel efficiency. But why exactly are diesel engines more efficient than gas engines?
More energy per litre
Three factors explain the superior performance of diesel engines. First of all, the fuel itself is denser and contains more energy, about 15% more by volume. It’s easier to measure the volume of a liquid than it is to weigh it, so the diesel engine profits from this by showing better values. When you compare energy contained per unit of mass, diesel and gas are virtually identical.
More efficient
The second factor is thermodynamic: the diesel engine’s cycle is more efficient than that of a gas engine, because combustion occurs at a higher temperature and is more complete. Why more complete? Primarily because the compression rate is higher. What’s more, the efficiency of a gas engine combustion cycle varies according to speed and load, while the efficiency of the diesel cycle is much more stable. On top of that, air intake in a gas engine is restricted by the throttle. And since a diesel engine doesn’t have one, it offers better efficiency even at low revs.
Less loss
Thirdly, driving style also makes a difference. Since diesel engines are rich in low-end torque (not to mention incredibly efficient), they aren’t so quick to boost rpms, right where gas engines lose their efficiency. Since low revs equal less friction loss, you get another boost in efficiency.
Compared to what, exactly?
In the end, all these little advantages add up to make a diesel engine 20 to 40% more efficient than a gas engine in the same vehicle. But, with this type of comparison, I wonder if it wouldn’t make more sense to compare two engines of equal power, as is usually done, or two engines that offer equivalent torque. Because, at equivalent torque, diesel has an even bigger advantage.
Let’s look at an “outside” example. It will give us a better perspective. We’ll take the Jeep Grand Cherokee: the diesel’s advantage (3.0L, 215 hp and 376 lb-ft of torque) over the V6 gas engine (3.7L, 210 hp and 235 lb-ft) is only 2.0L/100 km in the city and 1.0L/100 km on the highway. But if we compare it to the V8 of comparable torque (5.7L, 357 hp and 389 lb-ft), the difference is much more significant: 4.0 L/100 km in the city, and 2.0 L/100 km on the highway.
So, which comparison is valid? I asked some experts, but while I’m waiting for their answer, I’d like to hear what you think.
4 Comments
Comments
The short answer has two parts:
a) national capacity to refine diesel is falling behind demand
b) seasonal demand for furnace oil (pretty much the same stuff as diesel) is compounding this challenge
Andrew, can you add some detail?
Mike, I think I read somewhere there was also some basic economics at work here : the gasoline consumption is highly dependent on the fuel price, but the diesel demand is almost constant, no matter the price.
Some justify this by the fact that the trucking industry (one huge client for diesel) won’t stop because of high fuel prices, and can pass the expense down the client chain. So Big Oil can rise diesel price without too much harm on the volume.
Is it true ?
Guys, I am certainly no expert in the oil and gas business but have spent some time with professionals in this field over the past year through organization such as the CPPI (Canadian Petroleum Producers Institute). As I have come to understand the issue, Diesel, or “distillate” as it is refered in the petroleum world is in short supply in North America. The primary reason for this is the built refining capacity currently utilized by refiners in Canada.
When a barrel of crude oil is sent to a refinery in North America it is refined on average to 25% distillate and 75% gasoline. This is ratio is determined by the refining technology available through Canadian refiners. Contrast this with the refining capacity of Europe (50% distillate and 50% gasoline) and you begin to understand why we import large quantities of refined distillate (diesel fuel) into the North American market. We simply do not have the capacity. Apparently this weighting is shifting as refiners build out new diesel capacity however most of these changes will not take effect for about five years.
In the near term expect diesel pricing at the pump to remain at or near (slightly higher) than gasoline.
October 26, 2008 by adrien pelletier
why is diesel more money at the pumps