Every engine has a specific compression ratio. However, diesel and gasoline engines differ significantly when it comes to compression ratio. The first step in realising these differences is understanding what compression ratio is.
What Is Compression Ratio?
Compression ratio is the ratio of the cylinder volume when the piston is at the BDC to the volume when the piston is at the TDC. It is this ratio that determines the degree of compression of the air-fuel mixture before ignition.
Compression ratio in engines typically runs between 8:1 and 10:1. You may have heard that higher compression ratios, like 12:1 or 14:1, offer many benefits, including more power and greater fuel efficiency. That said, a higher compression ratio can also present significant risks, such as knocking. Which do you think offers higher compression ratio – petrol engine or diesel?
Compression Ratio in Petrol Engines
Petrol engines have spark-ignition and run on volatile fuels like petrol. Here, the air-fuel mixture is achieved post-compression. Air and fuel are mixed in a carburettor, and after compression, the mixture is ignited using an electric spark.
Petrol engines work based on the Otto cycle that involves two isochoric and two isentropic processes. Here are the phases in the combustion process in petrol engines:
- Intake
- Compression
- Ignition
- Exhaust
What is the compression ratio of petrol engines?
The compression ratio in petrol engines, over the last two decades, has always ranged from 8:1 to 12:1. However, there have been cases in the history of car production where automakers have gone higher than this ratio. Some of these cases include:
- 13:1 compression ratio in cars that were produced from 1955 to 1972; these cars were built to run on high-octane leaded gasoline that allowed higher compression.
- 14:1 compression ratio in some 2012 Mazda SkyActiv engine models; these engines have improved scavenging of exhaust gases, enabling them to realise this ratio using unleaded gasoline. The scavenging keeps the temperature levels in the cylinder low before the intake stroke.
- 14:1 compression ratio in Ferrari’s 2-door sportscar 458 Speciale
The reason why it’s not advisable to have a high compression ratio in engines that use low-octane fuel is that high CR could cause knocking. Also known as pre-ignition or detonation, this is when fuel auto-ignites and leads to uncontrolled combustion.
Knocking lowers the combustion efficiency and can cause severe damage if knock sensors aren’t installed to adjust the timing.
Compression Ratio in Diesel Engines
The creation of the diesel engine is attributed to Rudolf Diesel, after whom this type of engine was named. The invention was inspired by the inefficiency of the gasoline and steam engines at that time. Here are the four phases involved in the combustion cycle in diesel engines:
- Intake
- Compression
- Combustion
- Exhaust
Why do diesel engines have higher compression ratios?
Diesel engines have a higher compression ratio than their gasoline counterparts and don’t need spark plugs for ignition. Instead, fuel is introduced into the combustion chamber, where the air is compressed to a high temperature. This causes spontaneous ignition of the injected fuel.
What this means is that compression has to be high enough to increase the temperature of the air in the cylinder to a point where it ignites the fuel. For this reason, diesel engines are also called compression-ignition engines.
Air is compressed using adiabatic compression. Diesel engines only compress the air and not the fuel.
What is the average compression ratio of a diesel engine?
Direct injection diesel engines have compression ratios ranging from 14:1 to 23:1. Indirect injection diesel engines have compression ratios starting at 18:1 and go as high as 23:1. Unlike gasoline engines that use the Otto cycle, diesel engines operate on the Diesel cycle (compression ignition).
The diesel cycle consists of 4 processes as well: 2 isentropic processes, the constant volume process, and the constant pressure process.
Fuels with high resistance to auto-ignition are likely to cause late ignition. This can result in engine knock. While diesel engines have a higher peak combustion temperature compared to that of petrol ones, they eject less heat because of the greater expansion.
We hope that this simple guide has helped you understand more about the compression ratio in both diesel & petrol engines. You can read back on our article on 'What is compression ratio in car engines’ for some backgrounder on the subject.
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By Sam O.