Why Diesel Engines Have Higher Compression Ratios Than Gas Engines

Apart from the very curious, not many people ask why diesel engines, compared to gasoline, run higher compression ratios. The argument is reasonably straightforward and starts with fuel characteristics – diesel is less volatile than gasoline, heavier, and evaporates more slowly, which is why both engines are designed differently.

Okay, so how is all this connected to compression ratios, you ask? Since diesel vaporizes slowly, it requires more heat to get things started, as opposed to gasoline. And what better way to increase the cylinder temperature than by squeezing the air inside it even more. Mind you, this exercise is not exclusive to diesels, as a lot of uber quick sports cars feature high-compression gasoline engines. However, these often require higher-octane fuels. Why? Because regular pump gas can damage/destroy the engine through premature ignition, otherwise called engine detonation/knock.

Typically, diesel engines run compression ratios of 14:1 or higher, while gasoline engines have compression ratios ranging from 8:1 to 12:1, with Mazda's Skyactiv-G engine managing up to 14:1, which is pretty impressive for a mass-market powerplant designed to run on regular 87-octane pump gas. So, why can't you compress gasoline like diesel? To explain this, you need to first understand the chemistry behind the fuels themselves.

Have you ever wondered what a compression ratio means and why it matters? Put simply, an engine's compression ratio is defined as the ratio between the cylinder volume when the piston is at its lowest point (BDC or bottom dead center) and the cylinder volume when the piston is right at the top (TDC or top dead center). So, a ratio of 13:1 signals that the engine can squeeze the air-fuel mixture to a space that's 13 times smaller than what it originally occupied.

Flash point and auto-ignition temperatures are important considerations when you're talking about compressing fuels. Volatile fluids like gasoline are known for having low flash points, as they easily vaporize and ignite at relatively lower temperatures (-45 degrees Fahrenheit). The opposite is true for less volatile liquids like diesel. Auto-ignition temperature, on the other hand, is the minimum temperature at which a fluid spontaneously ignites without needing an external spark or flame.

Interestingly, diesel has an auto-ignition temperature of 410 degrees Fahrenheit, compared with gasoline's 536 degrees, which means diesel can ignite under compression alone without the need for an external ignition source. That should tell you why diesel engines don't have spark plugs. As you may know, diesels operate using compression ignition instead of spark ignition.

In short, gasoline engines don't require higher compression because the fuel itself is more combustible. However, having a higher compression ratio translates to more power, better thermal efficiency, and reduced fuel consumption, which is why diesel engines are often considered more fuel efficient. Although, diesel being more energy dense is also a contributing factor.