Gasoline and diesel engines are both four-stroke internal combustion engines, and from the outside they can look much the same. Inside, they ignite fuel in different ways, and that one difference shapes everything else: the weight, the torque, the fuel economy, the noise, the maintenance and the price.
Quick answer
A gasoline engine mixes fuel with air and lights it with a spark. A diesel engine compresses air until it is hot enough to ignite fuel sprayed into it. Diesel's higher compression delivers more torque and better fuel economy, with heavier construction and a higher price. Gasoline engines are lighter, cheaper, quieter and easier to start in the cold. Diesel suits long hours under heavy load; gasoline suits lighter, intermittent work.
In this guide
How each engine ignites fuel
In a gasoline engine, air and fuel are mixed before combustion, compressed to roughly 8 to 12 times their volume, and ignited by a spark plug. The mixture has to be controlled carefully: too much compression and it ignites on its own, which is the knock you hear in a struggling engine.
In a diesel engine, only air is compressed, typically 14 to 22 times, which heats it to several hundred degrees. Fuel is then injected directly into that hot air and ignites spontaneously. There is no spark plug and no throttle plate; engine speed is controlled by how much fuel is injected. Glow plugs, where fitted, only warm the chamber for cold starts.
| Characteristic | Gasoline engine | Diesel engine |
|---|---|---|
| Ignition | Spark plug | Compression heat |
| Compression ratio | About 8:1 to 12:1 | About 14:1 to 22:1 |
| Fuel energy per gallon | Lower | Roughly 10 to 15 percent higher |
| Fuel economy | Good | Better, often by 20 to 35 percent |
| Torque | Peaks at higher rpm | Strong at low rpm |
| Maximum rpm | High | Lower |
| Weight for the same power | Lighter | Heavier |
| Purchase price | Lower | Higher |
| Noise and vibration | Quieter, smoother | Louder, more vibration |
| Cold starting | Easy | Needs glow plugs, heaters or winter fuel |
| Maintenance | Spark plugs, carburetor or injection, fuel tends to go stale | Fuel filters, water separator, injectors; modern engines add DPF and DEF |
| Fuel storage | Months | Six to twelve months, longer with treatment |
| Typical equipment | Cars, mowers, portable generators, pressure washers, small pumps | Trucks, tractors, construction machines, large generators, marine |
Torque and power
Diesel engines make their torque at low engine speeds, which is why a tractor or a loaded truck can pull hard without screaming. The long stroke and high compression push the piston with more force per revolution. Gasoline engines rev higher and make their best power near the top of the range, which suits lighter machines that need to spin up quickly, such as mowers, chainsaws and pressure washers.
Fuel economy
Diesel wins on two counts. The fuel contains more energy per gallon, and the engine converts it more efficiently because of the higher compression and the absence of throttling losses. Over the life of a machine that runs for thousands of hours, the fuel saving can exceed the price difference between the two engines several times over. For a mower that runs 40 hours a year, it never will.
Durability
Diesel engines are built heavily to withstand their own compression, and they run at lower speeds, so they tend to last longer under heavy use. That is why they dominate equipment that works all day. Gasoline engines are not fragile, and a well-maintained small gasoline engine lasts for years, but they are engineered for lighter duty and lower cost.
Maintenance
A gasoline engine’s weak points are its ignition system and, on carbureted engines, the carburetor, which clogs when fuel is left to go stale. A diesel’s weak point is dirty or wet fuel. Fuel filters and the water separator need regular attention, injectors are expensive, and modern diesels in vehicles carry emissions equipment, such as a particulate filter and diesel exhaust fluid, that needs the right operating conditions. Our diesel maintenance schedule and small engine maintenance schedule set out the routine for each.
Cold weather
Gasoline evaporates readily, so a gasoline engine starts in deep cold with the right oil and a healthy battery. Standard summer diesel starts to cloud with wax at around 15°F (−9°C) and gels a little lower, and the engine needs heat to ignite it. Glow plugs, intake heaters, block heaters and winter-blend fuel solve the problem, but they are extra steps. See cold-weather diesel starting.
Safety of the fuel
Gasoline vapor ignites at temperatures far below freezing, which makes gasoline dangerous to store and spill. Diesel must be heated above roughly 125°F (52°C) before its vapor will ignite, so it is safer to store in quantity. That is part of why farms, fleets and building standby systems standardize on diesel.
Emissions
Diesels produce less carbon dioxide per mile or per hour of work because they burn less fuel, but more nitrogen oxides and particulates, which is why modern diesels carry exhaust aftertreatment. Gasoline engines produce more carbon monoxide and unburned hydrocarbons, controlled by catalytic converters on vehicles and simpler measures on small engines. Older small gasoline engines, such as those on some mowers and generators, are surprisingly dirty for their size.
Which one for the job?
| Equipment | Usual choice | Why |
|---|---|---|
| Lawn mower, string trimmer, chainsaw | Gasoline | Light, cheap, high-revving, easy to start |
| Portable generator for home backup | Gasoline or propane | Low cost, quiet inverter options, easy fuel supply |
| Generator running daily on a site | Diesel | Fuel economy and durability over long hours |
| Pressure washer | Gasoline | Compact, inexpensive, intermittent use |
| Tractor, skid steer, excavator | Diesel | Low-rpm torque, long service life, safer fuel |
| Pickup truck for towing | Diesel | Torque and fuel economy under load |
| Commuter car | Gasoline | Lower price, quieter, cheaper maintenance |
| Water pump, 2 to 3 inch | Gasoline | Portability and price; diesel for continuous dewatering |
Frequently asked questions
Why do diesel engines last longer?
Heavier construction to handle high compression, lower engine speeds, and the lubricating quality of diesel fuel all reduce wear. Many diesels in trucks and equipment run for several hundred thousand miles or many thousands of hours with routine maintenance.
Are diesel engines more expensive to maintain?
Routine service costs more: more oil, more filters, and more expensive parts when something fails. But diesels need no spark plugs or carburetor service, and their fuel economy often offsets the higher service cost for machines that work many hours.
What happens if you put gasoline in a diesel engine?
Gasoline has none of the lubricating quality that diesel injection pumps and injectors rely on, and it ignites far too early under compression. Even a small amount can damage the fuel system. If it happens, do not start the engine; have the tank drained and the system flushed.
