How much more efficient are diesel engines than gas powered engines? It’s widely been acknowledged that diesel engines get better fuel mileage than gas engines because they are fundamentally more efficient. How true is this? Let’s take a peak.
First, let’s look at efficiency argument. A diesel engine’s higher compression ratio and lean-burn combustion cycle provide an efficiency that no gas engine can currently match. Over the diesel’s operating range, the average “thermodynamic efficiency”—how much actual energy an engine gets from the fuel—is at least 15 percent better than a gas engine. Big advantage diesel?
Image credit: NRMA Motoring and Services
The reality is that this lead is shrinking fast. As emissions regulations become stronger, diesels are slowly losing their edge in the efficiency race. A big issue is that most of today’s diesel engines need scrubbers that clean up the dirty diesel exhaust. And these systems happen to crimp the overall efficiency of the system due to their design. They soak up some of that extra energy that the diesel fuel provides.
And gas engines continue to improve. Once-exotic efficiency-enhancing hardware such as variable camshaft timing, direct fuel injection and turbochargers have become commonplace on gasoline engines. And there’s more on the way, like lean-burn combustion and homogeneous charge compression ignition. But while their gasoline powered cousins continue to improve, don’t expect the diesel engine to stay static. Around the world, engineers are working hard to improve the efficiency of diesel engines even more. The gains will come from hardware such as variable valve timing and independent cylinder combustion control, as well as improved after-treatment systems for diesel exhaust.
As you can see, the race is far from called. What is interesting is that while Diesel engines are becoming better accepted over here in the US, they make up over 50% of sales in Europe. The reason for this is probably more due to an open mind about diesel engines than pure economics, though. American manufacturers have made some very poor diesel engines in the past, such as the infamous 350 Oldsmobile Diesel in the 1970s, and this has slowed American acceptance.
Source: East Hills Chevrolet