DI Combustion Chamber Design


Large slow running DI engines (under 1500 rpm) have 'open chambers' where the piston has a very shallow dish shaped with a bump in the middle. Air movement is minimal and the multi hole injectors (often 8-12 hole) are set to inject a fine mist into the dish. Due to the size of the combustion chamber sufficient air is present to supply the fuel with oxygen and the fuel combusts before it contacts surfaces of the combustion chamber.


With smaller higher speed engines it is not possible to produce injectors with the multiple very small holes that would be necessary to provide sufficient fuel distribution. The number of holes is therefore reduced (3-5 hole). To achieve better fuel/air mixing air turbulence is induced within the combustion chamber.

Incoming air is set into rotation by the inlet valve being positioned to one side of the cylinder head. The air rotates around and down the cylinder as it is pulled in. This rotational force is sometimes increased by having a helical induction port passage where the air is guided into a semi-vortex swirl around the valve stem on its way into the cylinder.

The piston has a bowl in its crown. As the piston approaches the top of the cylinder the rotating air is forced into the piston bowl. The rotational force is magnified by the reduced diameter of the piston bowl. Thin, deep bowls have a higher swirl rate.

At the same time air squish is initiated as the air is forced from above the piston crown in towards the centre of the piston. The squish forces meet at the centre of the cylinder and oppose each other being forced downwards into the bowl where they follow the bowl profile being led into a horizontal swirl.

Simple bowl showing squish action - also described as torodial.

In order for the fuel and air to mix the fuel injection is set to spray against the wall of the piston bowl. The drag created by this contact stops the combusting fuel spray being dragged with the swirl and allows the fuel to contact with fresh oxygen. With thin bowls the spray does not have to penetrate as far to reach the combustion chamber wall so a wider more fine injection can be utilised that will mix more readily with the fuel.

With the quickest engines large air movement is required to give a fast mix. The fuel spray has to be more penetrating to overcome the increased air forces and the injection event rate is increased so that the fuel charge is swiftly delivered and has longer to mix and combust.

A trick used by several engine manufacturers is to give the bowl a lip to prevent the air squish motion pushing fuel above the piston crown, so that the majority of the fuel charge is mixed and burnt within the bowl. The lip also creates further micro turbulence within the bowl. Engines with this re-entrant lip design include VW/Audi tdi engines (thanks to John O for this info), Perkins Prima, Perkins 42482 and some Isuzu square (when looking from above) bowl chambers. The square chamber produces micro turbulence from its rounded corners which provide superior air-fuel mixing.

Perkins Squish Lip Re-Entrant Bowl Piston

Isuzu Square Bowl Piston

Chevy Duramax Piston (Cheers JMJ)
The Elsbett Piston

The Elsbett engine has a deep bowl which has a slight lip. The main difference is that the fuel charged is injected in such a manner as to 'blend perfectly with the air' and combust within a central core of hot air, not contacting the chamber walls, which is necessary for good air/fuel with other designs examined.

The MAN M system (film) combustion chamber

A similar shaped piston bowl to the Elsbett system. With this design the fuel is injection is directed onto the chamber wall where it spreads as a film, combusting as the film evaporates due to the heat of the piston. The heat of the piston has to be within a temperature range to achieve fuel evaporation without causing thermal decomposition and carburizing of the fuel.

Lessons to be learnt for Veg Oil

To summarise I would presume:-

Quick injection rate gives more time for the oil to begin to combust.

Chamber shapes that create greater micro turbulence - the Isuzu square chamber would appear superior in this respect - should provide better fuel air mixing and faster combustion.

Deep re-entrant chambers should be more effective than shallow chambers.

Comments, suggestions or observations will be gratefully received from those with relevant experience or information.





'Advanced Engine Technology' by Heinz Heisler ISBN 0-340-56822-4

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