Going back to better spark

Spark ignition engines are back in vogue - we look at how the SIDI system works

Published:  20 March, 2013

If you looked at the line-up of the new generation Fiat Panda last year, you might have noticed that there was something missing from the range - namely, a diesel engine.

The Fiat group played a significant part in what we now call the 'modern' diesel engine - i.e. with turbocharger and a common rail. Indeed, the Multijet engine set new standards in efficiency and driveability when it was first introduced in the Alfa 156 back in the 1990s.

However, emissions standards and start/stop systems cause problems for vehicle designers. While it certainly isn't the death knell for diesel engines, some manufacturers such as Fiat prefer to invest in developing the ignition systems on petrol (or 'gasoline' as engineers prefer) engines.

One of these technologies is known as 'Spark Ignited Direct Injection' or SIDI. This makes the most of cruising speeds by using a 'stratified charge' while during heavier load conditions, the engine management system will employ 'stoichiometric' mode.

Stoichiometric mode is used for higher engine loads. Fuel is injected during the intake stroke, creating a 'homogeneous' fuel-air mixture in the cylinder. From the stoichiometric ratio, such a clean burn also results in a much reduced exhaust emission.

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