Why didn't German engines use 4 valve cylinders?

[wiking85]

Reading recently about the Jumo 213J that was going to introduce 4 valve cylinders to the engine instead of the standard 3 valves, I have to wonder, why didn't the Germans used 4 valve for their cylinders in all engines from the beginning? The greater efficiency of the 4 valves is pretty apparent when looking at the volumetric efficiency, so why was this a late war development? Am I missing something, like was this only a problem of Jumo engines are something?

 

[Shortround6]

The DB engines did use 4 valves. With supercharged engines the need for 4 valve heads is reduced ( not eliminated) but the usual restriction on engine power in an aircraft engine is the strength of the engine. It was all to easy to get the engine to make more power than it would stand up to.
Some things do not scale well but a lot of 1939-40 aircraft engines made more power per pound than some 1939 Formula I Grand Prix engines and they did it on 87-100 octane gasoline, not exotic concoctions of methanol, nitrobenzol, acetone, and sulphuric ether.

That said, later Russian V-12s moved from the Hispano 2 valve head to a 3 valve and then to a 4 valve. But they needed stronger crankshafts, crankcases, piston rods and other parts to stand up to the power.

 

[wiking85]

Why was Jumo behind then? Just the power being too much for the crankshaft?

 

[Shortround6]

It is hard to say if they were behind. A lot of things were done by trial and error and bits of formula. Actual airflow through ports, valves and manifolds was seldom measured or understood as it is today. And there are mechanical considerations. Some old top fuel dragsters used heads with slightly smaller valves than maximum because the biggest valve heads had weak points between the valve seats or between the valve seat and cylinder edge and would crack.
A Jumo 213 uses the same bore and stroke as a 211 yet gained around 700lbs in weight. In large part to beef up the engine for the higher power outputs.

Is the extra power the 4th valve would allow worth the cost, in time-money-maintence?

Everything is a trade off and perhaps the 213 could use the same tooling/casting patterns ( or some of them) as the 211 cylinder heads.

 

[wiking85]

Is the extra power the 4th valve would allow worth the cost, in time-money-maintence?

Everything is a trade off and perhaps the 213 could use the same tooling/casting patterns ( or some of them) as the 211 cylinder heads.

Apparently Jumo thought so, as they were trying to introduce it by the end of the war, but allied bombing prevented the changeover before it could happen (just as it gutted production of the 3 valve model in 1944).

 

[wiking85]

Is the extra power the 4th valve would allow worth the cost, in time-money-maintence?

Everything is a trade off and perhaps the 213 could use the same tooling/casting patterns ( or some of them) as the 211 cylinder heads.

Apparently Jumo thought so, as they were trying to introduce it by the end of the war, but allied bombing prevented the changeover before it could happen (just as it gutted production of the 3 valve model in 1944).

 

[davebender]

DB605A used 4 valves per cylinder.
DB605D used 3 valves per cylinder.

I have no idea why DB switched to 3 valves but it certainly didn't hurt power output.

 

[Ivan1GFP]

More valves, especially poppet valves isn't necessarily better.

All the open valves inside the cylinder are obstructions to flow and flow is the main objective.
Also with 4 valves instead of 3, you must use smaller ones.

If numbers of valves were so much better, why not use 5 valves per cylinder?
It has been done before.

I suspect there is also a significant reduction in strength as the exhaust valve gets smaller, especially since typically they are hollow inside for sodium cooling.

- Ivan.

 

[Shortround6]

Apparently Jumo thought so, as they were trying to introduce it by the end of the war, but allied bombing prevented the changeover before it could happen (just as it gutted production of the 3 valve model in 1944).

They were using it on the 213E after building thousands of earlier 213 engines. Maybe they had solved other problems, maybe it was needed for the power they wanted from the "E". Point is that there is a lot going on and without knowing why the designers/company did something it is hard to really criticize 70 years later. Exceptions to this may be made for notable gaffs like the Armstrong Siddiley Tiger ( no middle bearing), the Jumo 222, the Wright Tornado and a few other "what were they thinking " engines.

I would note that the P&W and Wright radials used two valves per cylinder. However their cylinder heads allowed for widely splayed valves and a considerable angle to the bore which allowed for some rather larger valves. Some 4 valve engines used parallel valves in line with the bore which restricted actual valve area.

Without knowing more about the ports and manifolds it is hard to say if the valves were the restriction or not.

 

[wuzak]

Some things do not scale well but a lot of 1939-40 aircraft engines made more power per pound than some 1939 Formula I Grand Prix engines and they did it on 87-100 octane gasoline, not exotic concoctions of methanol, nitrobenzol, acetone, and sulphuric ether.

You started me off on a tangent until I reread this statement.

Namely that the power to weight of an aero engine in WW2 was comparable to an F1 GP engine. I was going to point out that the current F1 engines weigh 95kg (mandated minimum) and put out 720hp (conservative estimate). As a 2 stage Merlin weight ~750kg we can figure the equivalent power to weight would be ~5700hp! And that at the height of the V10 era the engines were making 900hp+ and weighed 80-85kg - an equivalent of 7940-8440hp for a 2 stage Merlin.

Then I realised that you said a "1939 F1 engine". Such a thing didn't actually exist (F1 was formulated post WW2), but I get that you mean GP engines.

The best of the 1939 GP engines was probably the Mercedes-Benz M163, which raced in the W154 GP car. The M163 was a development of the M154 V12, one of the changes being a move from two parallel Rootes type superchargers to a 2 stage system, still Rootes type. I have a book which has the weights of many racing engines, but it is in storage. The M163 made ~475hp, and the whole car weighed 910kg (1939). So, the aero engines are probably quite close to the power/weight ratio, despite the longevity requirements.

 

[hrandy]

Did the 605D really have a three valve head? I can't find any reference to this type of significant change in any of the english language books and articles I have.

 

[Denniss]

All 605 had 4 valves per cylinder, no idea where this 3-valve theory comes from.

Junkers had once decided to use 3 valves per cylinder for the 210 and took this over for the 211. There was no real need/pressure from anyone to change this to a 4-valve-system, introducing it would also disrupt the production for some time.

 

[davebender]

Is this source wrong?
Identifying DB605AS vs DB605D variants- part 2

Although similar externally to the earlier A-series motors, the D had numerous internal changes, among which was a redesigned head and valve train; the A model had four valves per cylinder (two inlet, two exhaust) while the D had a single inlet valve and two exhaust valves (source: JaPo, p. 81).

 

[Ivan1GFP]

A single Inlet and two Exhaust valves is a VERY unusual configuration.
There are generally more or larger inlet versus exhaust because cylinder pressure after combustion helps scavenging.

- Ivan.

 

[Denniss]

This is an older page that hadn't ben updated for a long time (or this section had been forgotten). The 3-valve configuration is plain wrong - DB would have had to change too much stuff inside and outside of the engine to go from 4 to 3 valve.