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May "Fireball"

May "Fireball"

RE: May "Fireball"

Sure. It was over optimised for one parameter, and sacrificed too much of other important characteristics. Maybe Marquis will be able to dig up facts (I am relying on 20 year old memories), but I vaguely remember it caused EXCESSIVE turbulence in the chamber. This dragged in the cool mixture from the cylinder wall, and so (in particular) HCs got worse.

Don't shoot me if I'm wrong, at the time my section were testing 4 different engines, at least.


Greg Locock

RE: May "Fireball"

Thanks, Greg. You're right the turbulence was very high, but according to the test data of a Passat prototype head in the papers written by Michael May (I. Mech. E. C204/79 & C97/79 + SAE 790386 / 1979) the NOx emissions were decreased by a factor 3, the CO by 4 and the HC were roughly the same as the production Passat while the fuel consumption was reduced up to 32%.

Marquis, are you here? Any info?



RE: May "Fireball"

Sorry about the delay Aorangi

What Greg says is essentially correct, but along with other issues.
The combustion chamber concentrated around the exhaust valve  along with the slight bowl in the piston lead to a high suface to volume ratio chamber.

This leads to a relatively poor BSAC (break specific air consumtion) value- this is a measure of how an engine burns for a given air flow. This didn’t matter too much for the engines over here that ran a 12.5:1 CR ( I forget what these engines ran Federally)- as the high CR  goes some way to compensate by getting more BMEP for a given airflow.

Now poor BSAC is quite common in very oversquare engines even today-but these engines usually have large valve area per cylinder size, good flowing ports and are usually pent roof these days. So what they don’t have in terms of BSAC efficiency they make up for by having good Volumetric efficiency (air flow).
The May head doesn’t have this luxury, It was a two valver for starters, and had these essentially parallel positioned valves fairly small ( compared to a hemi, say) where the ports were optimised to induce chamber charge motion (axial swirl if memory serves). This meant the ports didn’t flow as well for outright flow performance for an already relatively small valve. This isn’t a problem on a “lazy cubes” low specific output type engine. But when Specific output became a priority- like on our V8 , valve area also became a priority.

Then as emissions laws got very stringent, world wide, and fuel octane ratings tumbled compression ratios had to be lowered.
I believe the Euro spec 6.0 litre last of the line V12s (that ran Zytek engine management) were running on 10:1 CR- but don’t quote me on that! The CR had to be dropped for fuel octane AND Nox regs now.
Again the V12 chamber was an odd beast in that it resisted knock fairly well compared to it’s 2 valve contemporaries, but had SLOW 10-90 burn rates. Even so the early 5.3 litre 12.5 CR engines WERE knock limited at full load- and the main benefit of the high CR was clearly at part load. The slow 10-90’s didn’t bode well for the stringent modern emissions laws. More to point, with the poor BSAC and without the high CR the benefits of the chamber were looking marginal.
Add to that that modern conventional catalyst cars can’t run lean at part load, something the V12 was designed to do from the outset .

The V12 along with the staright six (which ran along the same lines)were made at Radford, a plant that it was decided to close so this also had a bearing on things.

The high specific horsepower requirement for the new XJ/XK series engines sealed the V12s fate.

It saddens me when I drive one of these beasts, and am in awe of their phenomenal refinement!

The V12 is a very cheap engine to het hold of second hand an a few of us have played with the idea of whether it would be possible to use the straight six 24 valve heads from the AJ6/AJ16 for cheap after market power!

They have the same bore centres. The other head could be flipped around, and a cam chain drive would have to be implemented on the other end of the head. However, the bore sizes themselves are slightly different and the V12 is wet linered and has studs in the block which the head nuts down onto. In contrast the staight six Jag engine had head bolts that ran through the cam caps and I wouldn’t be surprised if the bolting/fastening position was different.

Oh well, was a good idea I thought!

RE: May "Fireball"

Thank you very much , Marquis for your extremely interesting post about your first hand experience with the Jaguar V12 and the Fireball combustion chamber. You don't have to apology about the short delay, it was well worth the wait ! I was just impatient to know whether you saw that thread because I met Michael May the other day and we talked a lot about the Fireball chamber. He told me they obtained a bsfc as low as 140 g/hp/h at the Continental Aircraft Engines company on a prototype with large cylinders. I'm also quite impressed with the data provided in the papers he lent me.

I suspected there was some emissions control problems. As the Fireball chamber was designed to burn lean mixtures, it lost a great part of its advantages when the catalyst was fitted. Still, according to the tech data I dug in the CR was 11:1 in 1997 on the latest Jag V12 produced. Michael May find it a pity that the last bit of power must be extracted from each engine model for commercial purposes instead of trying to get the best possible fuel efficiency.

On the papers I read there's no mention of an intake port generated swirl. I understand that a lot of squish was generated around the intake valve area and from there the charge was directed by a tangential groove like recess (or guide channel) into the head and leading the to the main part of the combustion chamber under the shorter exhaust valve where it swirled around, a little bit like in a Ricardo Comet swirl chamber, but horizontally. Well, you know it better than me, I just describe it for the other readers.

I'm wondering whether this concept couldn't be revived today, used together with a Denox catalyst  – like in direct injection stratified charge engines. Of course, specific output would be less than with a 4 valves head, but production cost would be much less.

Thanks again,

RE: May "Fireball"

You can bias the burn to the exhaust side on a 4-valve engine fairly easily with both chamber and piston design. This can give you an excellent flow value, but probably more surface area than you would get out of a more conventional pent roof design and flat-top piston. I believe there is value to doing this.

There is also little doubt that a 4-valve engine can produce some very good mixture motion, especially when valve events are staggered. So the May design has some of the good design ideas, but is probably not the optimum model of the concepts involved. To be fair to May, the head was a re-design from the start. Had he been able to design a head layout from scratch, he may have achieved more.

The biggest problem I see with a current-day application is the need for 14:1 fuel ratios to make the cat happy.


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