dished vs. flat top piston.

[SMOKEY44211]

Most OEM Gasoline engines use a dished piston. I'm assuming that there is an advantage in reduced ignition advance timing requirements and/or reduced emmissions. I've asked a number of custom piston manufactures but no one seems to know. Any thoughts or information? -----Phil

 

[malbeare]

it probably has more to do with less machining time than anything related to engine performance . maybe the machining for valve cutouts is not necessary with dished pistons.
malbeare

A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare

http://www.sixstroke.com

 

[evelrod]

Most dished pistons are a bit heavier, some a lot. Many of the applications utilize a 'flat' cylinder head design thereby eliminating a lot of machine work, e.g., the Kent Ford 116E type (Cortina, Fiesta, etc). Some use a dished piston to lower the CR when the overall engine displacement is increased beyond the ability to increase the combustion chamber sufficiently to compensate, e.g., BMC/Leyland "A" series (Mini, Sprite, etc.)
I'm not sure that the practice is anything more than a cost saving device for the mfgrs as in a race/performance design, the practice is non existent...save for a couple extreme and, rare cases.

Rod

 

[IceStationZebra]

I assume that in many cases the design helps with swirl and/or quench resulting in a more ideal combustion. I think that quench in particular, depending on the chamber design, would help slightly to reduce ignition timing.

ISZ

 

[BrianPetersen]

Concentrating the air/fuel mix towards the center of the chamber is generally regarded as a good thing to be doing. Dished piston (and a shallow/compact chamber in the head) does that. Crowned piston does the opposite.

 

[mattsooty]

Yes, part of the rationale for using dished pistons is to introduce squish in the end gas regions.

Since the edges of the combustion chamber are usually the zone in which detonation is initiated, accelerating these gases towards the centre of the chamber (where the plug is generally located) means that more ignition advance can be used without being knock limited. There is also a corresponding increase in flame kernel development and reduction in burn time (and improvement in combustion stability) due to this mechanism.

That said, issues arise when the geometry is such that at lower piston speeds quench occurs, with a corresponding increase in pollutants due to incomplete combustion of the charge.

MS