Why Diesel Engines Have Higher Compression Ratios Than Gas Engines – Jalopnik

Apart from the very curious questions, not many people wonder why diesel engines, compared to gasoline, have higher compression ratios. The argument is fairly simple and starts with the fuel properties: diesel is less volatile than gasoline, heavier and evaporates more slowly. Therefore, both engines are designed differently.

Okay, so how is this all connected to compression ratios, you ask? Because diesel evaporates slowly, it needs more heat to get things going, unlike gasoline. And what better way to increase cylinder temperature than by pushing the air inside it even further. Mind you, this exercise is not exclusive to diesels, as many super-fast sports cars are equipped with high-compression petrol engines. However, these often require higher octane fuels. Why? Because normal pump gas can damage/destroy the engine due to premature ignition, also called engine detonation/knocking.

Typically, diesel engines have 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 achieving a compression ratio of 14:1, which is quite impressive for a mass engine designed to run on regular 87 octane pump gas. So why can’t you compress gasoline like diesel? To explain this, you first need to understand the chemistry behind the fuels themselves.

Flash point and auto-ignition temperatures are important considerations when talking about compressing fuels. Volatile liquids such as gasoline are known to have low flash points because they easily evaporate and ignite at relatively lower temperatures (-45 degrees Fahrenheit). The opposite is true for less volatile liquids such as diesel. The auto-ignition temperature, on the other hand, is the minimum temperature at which a liquid spontaneously ignites without the need for an external spark or flame.

Interestingly, diesel has an auto-ignition temperature of 410 degrees Fahrenheit, compared to gasoline’s 536 degrees, meaning diesel can only ignite under compression 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 use compression ignition instead of spark ignition.

In short, gasoline engines do not need higher compression because the fuel itself is more flammable. However, having a higher compression ratio translates into more power, better thermal efficiency and better fuel economy. That is why diesel engines are often considered more economical. Although diesel’s higher energy density is also a contributing factor.