Gas vs Diesel Engines: Differences, Pros & Cons
A gas (gasoline) engine ignites its air and fuel mixture with a spark plug. A diesel engine compresses air until it is hot enough to ignite diesel fuel sprayed into the cylinder, with no spark plug. Thanks to that much higher compression and a denser fuel, diesels make strong torque at low RPM and use less fuel under heavy load, but they cost more and weigh more. Gas engines are lighter, rev higher, cost less to buy and need less emissions equipment. Which is better depends on how hard and how often the engine has to work.
Gas vs diesel engines at a glance
| Gas engine | Diesel engine | |
|---|---|---|
| Ignition | Spark plug | Heat of compression (glow plugs help cold starts) |
| Compression ratio | Roughly 8:1 to 12:1 on most engines | Roughly 15:1 to more than 20:1 |
| Torque | Peaks higher in the rev range | Strong at low RPM |
| RPM range | Revs higher | Lower maximum RPM |
| Fuel economy under load | Lower | Better; diesel fuel holds more energy per gallon |
| Purchase price | Lower | Higher |
| Weight | Lighter | Heavier, with a stronger block and parts |
| Emissions equipment | Catalytic converter | Modern road diesels add a particulate filter and often DEF (urea) injection |
| Typical uses | Cars, light trucks, performance builds, small boats and outboards | Heavy trucks, frequent towing, commercial vehicles and larger boats |
How gas and diesel engines ignite their fuel
A gas engine draws in air, mixes it with fuel (in the port, in the cylinder or through a carburetor), compresses the mixture and fires it with a spark near the top of the compression stroke. A throttle plate controls power by limiting the air the engine can draw in.
A diesel draws in only air and compresses it much harder, which heats it well above the ignition point of diesel fuel. A high-pressure injector then sprays fuel into the hot air and it ignites on its own. Power is controlled mainly by how much fuel is injected. Because there is no spark to time, diesels rely on precise injection timing, and most modern diesels are turbocharged.
Torque, towing and the gas vs diesel truck question
Diesels are usually long-stroke, low-revving engines, and with turbo boost they make very high torque at low RPM, which is why heavy-duty pickups offer them for towing. Large gas V8s narrow the gap: Ford’s 7.3L Godzilla gasoline V8 is rated at 430 HP and 475 lb-ft in Super Duty pickup tune, in trucks that are also sold with a diesel. A gas truck costs less to buy and suits lighter or occasional towing; a diesel pays back its higher price when it tows heavy loads often and covers many miles. For what the torque and horsepower figures mean, see horsepower vs torque.
Fuel economy, running costs and maintenance
- Fuel economy: diesel fuel contains more energy per gallon and diesels are more efficient under load, so they usually go farther per gallon, especially when towing.
- Running costs: diesel fuel prices vary against gasoline, and modern road diesels use DEF (diesel exhaust fluid), an extra running cost.
- Maintenance: diesels need clean fuel, fuel filters and water separators, and care of the emissions system; gas engines need spark plugs and ignition parts. Both need regular oil changes.
- Repairs: diesel injection systems and turbochargers are expensive to repair, while gas engine parts are usually cheaper and easier to find.
Gas vs diesel boat engines and diesel outboards
On larger boats, inboard diesels are common for long-range cruising and commercial work: they are economical under continuous load, and diesel fuel is far less volatile than gasoline. Runabouts, smaller boats and most outboard-powered boats use gasoline, because gas engines are lighter, cheaper and make more power for their weight. Diesel outboard motors exist, but they are a niche product: heavier and much more expensive than gasoline outboards of similar power, and aimed mainly at commercial and military users. For engine placement, see inboard vs outboard motors.
What we sell
Every engine we sell runs on gasoline or electricity. Our crate engines from BluePrint, GM Performance and Ford Performance are gasoline engines, with compression ratios in their specification tables such as 9.4:1 for the BluePrint SBF 302 EFI, 10.3:1 for the GM 6.0L LS364/450 and 12:1 for the Ford Aluminator XS. Our Suzuki and Honda outboards are gasoline four-strokes, and the Torqeedo Travel is electric. We do not sell diesel engines.
More reading: engine vs motor.
Frequently asked questions
Is a diesel engine better than a gas engine?
Not for every job. A diesel is better for heavy, frequent towing and long hours under load. A gas engine is lighter, cheaper to buy and better suited to cars, light trucks, performance builds and small boats.
Why do diesel engines make more torque?
Diesels run much higher compression, usually have a long stroke, and most are turbocharged, so they build high cylinder pressure at low RPM. Diesel fuel also carries more energy per gallon than gasoline.
Are there diesel outboard motors?
Yes, but they are uncommon, heavy and expensive, and mostly used commercially. Almost all recreational outboards are gasoline or electric. The outboards we sell are Suzuki and Honda gasoline four-strokes and the Torqeedo Travel electric outboard.
What happens if you put diesel in a gas engine?
The engine will run poorly or stall, because diesel does not ignite properly with a spark. Do not keep running it: have the tank and fuel system drained and flushed before restarting.
Is the Ford 7.3 Godzilla a gas or diesel engine?
It is a gasoline V8. It shares its displacement with Ford’s older 7.3L Power Stroke diesel, a completely different engine. See the 7.3 Godzilla engine guide.
Do diesel engines last longer than gas engines?
Often, yes. Diesel engines are built with heavier blocks, crankshafts and bearings to handle their high compression, and they do most of their work at lower rpm, so truck diesels commonly run longer between overhauls than comparable gas engines. Maintenance, load and fuel quality matter more than the fuel type, and a diesel’s injection and emissions parts are expensive when they do wear out.
Why are diesel cars less common than gas cars?
In the US a diesel car usually costs more to buy, diesel fuel is often priced above regular gasoline, and modern diesels need emissions equipment such as a particulate filter and DEF (diesel exhaust fluid) that add cost and upkeep. Their fuel economy and torque pay off mainly when towing or covering long distances, which is why diesels stay common in heavy pickups and work trucks.
Related: what is a crate engine?, how to break in a new engine, 2 stroke vs 4 stroke outboards, compression ratio, all guides.
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