Diesel vs Petrol Engines: Spark or Squeeze?
Petrol engines light their fuel with a spark; diesel engines light it by squeezing air until it is hot. Here is what that one difference changes.
In this article
The main difference between a petrol engine and a diesel engine is how the fuel catches fire. A petrol engine uses a spark plug to ignite a mixture of fuel and air. A diesel engine has no spark plug: it squeezes air until the air is hot enough to set fire to fuel sprayed into it. Almost every other difference, from efficiency to noise to exhaust, follows from that choice.
How a petrol engine lights its fuel
A petrol engine is a spark-ignition engine. Fuel and air are mixed, drawn into a cylinder and squeezed by the piston. At a precisely timed moment the spark plug fires and the mixture burns, pushing the piston down. The spark's high voltage comes from an ignition coil, and in modern cars an electronic control unit decides when it fires.
The mixture is already in the cylinder while it is being squeezed, so there is a limit to how hard it can be compressed. Squeeze it too much and it may ignite by itself before the spark, an unwanted event called pre-ignition. This is why petrol engines run at lower compression than diesels. Petrol is also sold with an octane rating, which describes its resistance to burning too early.
The first practical petrol engine was built in 1876 by Nicolaus Otto and Eugen Langen in Germany, according to Wikipedia's article on the petrol engine. Most petrol engines still follow the four-stroke cycle named after Otto.
How a diesel engine lights its fuel
A diesel engine is a compression-ignition engine. Only air is drawn into the cylinder. The piston squeezes it to a compression ratio typically between 15:1 and 23:1, which heats the air strongly. Near the top of the stroke, fuel is injected, and the hot air ignites it. No spark is needed.
Because fuel arrives only at the last moment, pre-ignition is not a problem, so a diesel can use much higher compression. The Energy Education encyclopedia makes the same point: a petrol engine at diesel-level compression would ignite its fuel-air mixture too early.
Why diesels tend to be more efficient
Squeezing the gas harder lets an engine turn more of the fuel's heat into movement. Wikipedia's diesel engine article gives up to 43 percent efficiency for passenger-car diesels, up to 45 percent for large truck and bus engines and up to 55 percent for large two-stroke marine engines. Its petrol engine article puts the thermodynamic efficiency of petrol engines at about 20 to 30 percent.
These are best-case figures. The diesel article notes that average efficiency over a real driving cycle is lower than the peak, giving an example of 37 percent average for an engine with a 44 percent peak.
The Energy Education project compares a 2014 Volkswagen Golf TDI (diesel) with the petrol version of the same car. It reports the diesel as 8 miles per gallon better in combined driving and 12 better on the highway. That is one model pair in one year, not a rule for every car.
Torque and starting
Diesels generally make more torque, the twisting force that gets a vehicle moving, for their size. The same source lists 236 lb-ft for the 2015 Golf TDI against 185 lb-ft for its petrol sibling. Starting a diesel is harder work for the starter motor, which must turn the engine against high compression, so diesels need more battery power to start.
Emissions
Burning diesel produces carbon monoxide, unburned hydrocarbons, particulate matter (tiny soot particles) and nitrogen oxides. Wikipedia reports that exhaust treatment can remove about 90 percent of these pollutants and that road diesel has been sulfur-free since 2003. It also notes that particulate matter is classed as a carcinogen or probable carcinogen. The Energy Education project says diesel tends to produce more particulate matter than petrol.
Rules differ by country and change over time, so check the current regulations where you live. This article gives general information only.
Where the idea came from
Rudolf Diesel published his proposal, Theory and Construction of a Rational Heat Motor, in 1893. His first idea, with compression at constant temperature, would not work, and by June 1893 he had switched to a different cycle. His first prototype fired for the first time on 10 August 1893. A later test on 17 February 1897 rated an engine at 13.1 kilowatts with an effective efficiency of 26.2 percent, and the first commercial diesel engines were installed in 1898.
The short version
- Petrol: spark ignition, lower compression, efficiency of about 20 to 30 percent.
- Diesel: compression ignition, compression ratios of about 15:1 to 23:1, higher efficiency.
- Diesel engines make more torque for their size but need heavier starting effort and exhaust treatment for particulates and nitrogen oxides.
- Both burn fuel in a cylinder to push a piston; only the way the fire starts differs.
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