SPARK VS COMPRESSION
Before we can understand the benefits and advantages delivered by different types of fuel, we must first look at how it is used by the host engine. In short, there are two primary types of combustion engine: spark ignition and compression ignition.
Spark ignition is the label given to a petrol engine. These powerplants ignite the compressed air/fuel mixture in the combustion chamber through spark plugs. Compression ignition is a way of describing diesels, where spark plugs are absent. Instead, the engine runs a very high static compression ratio and the fuel (diesel) is of a low octane rating, which allows ignition without a separate spark required to initiate combustion.
Another major difference worth noting is what effect fuelling conditions have. Most of you will know that a petrol engine running lean – especially under load – can lead to high operating temperatures. This condition encourages dangerously hot exhaust gases, leading to severe engine damage. It is with this in mind that many tuners allow their modified engines to run deliberately rich in a bid to promote reliability.
As far as diesel engines are concerned, the leaner they run, the cooler they become. In fact, all diesels run lean in factory specification, which is one of the reasons they’re generally more fuel efficient than petrol engines. On the downside, increasing the air/fuel ratio to a richer mix would cause a diesel engine to produce unsightly black smoke (which is what many think of when asked to bring a diesel to mind). Lean running isn’t ordinarily a problem for diesel engines, meaning they can run high boost pressures when compared to those produced by engines making use of spark ignition.
The lesser-spotted VXr220 Turbo (only sixty were built) isn’t as thirsty as you might think