109 e3b against spitfire II

[TomcatViP]

Originally Posted by Viper2000

OTOH, because the supercharged engine is smaller, it has less non-cooling drag, and so you'd expect to cruise faster, which helps to make the radiator smaller.

Funny that Merlin Vs DB tell the opposite.

Seems you forgot one parameter : rpm and charging raise the strain and the temp with negative consequence on efficiency: try to win the 24h Le Mans race with a 2L engine and then jump in 7.0L 'vette

To put it in perspective : there was no successful post war Merlin engined airliner. But lot of with P&W primitives big radials

[TomcatViP]

Originally Posted by Viper2000

If direct injection really was so great for piston aero-engines, the chances are that the Allies would have adopted it immediately post-war when all Axis technology was theirs for the taking. The fact that they didn't do so speaks volumes.

Sry but you are bypassing economics realities : the Industrial war machine was in such a strain at the end of WWII that minimal change in production were made where there was not strategical importance in order to downsize the level of investments. Many non-allied advanced tech were simply rejected in face of this.

Civil Aviation (the only one still interested in piston engine at the time) loose for long Injected eng, Fadec (without D and E ), canards foreplane etc.. Some of the very much "advanced" tech that was rushed back on the GA shelves as "new" products in the late 80's and 90's.

The conclusions you give does not convince me - Sry I am duplicating earlier comments of very good quality

[Viper2000]

Originally Posted by TomcatViP

Funny that Merlin Vs DB tell the opposite.

Seems you forgot one parameter : rpm and charging raise the strain and the temp with negative consequence on efficiency: try to win the 24h Le Mans race with a 2L engine and then jump in 7.0L 'vette

To put it in perspective : there was no successful post war Merlin engined airliner. But lot of with P&W primitives radials

rpm isn't especially interesting. Piston speed is generally a better metric.

Liquid cooled engines run colder than air cooled engines, and actually one of the main problems for the Merlin was over-cooling of the charge during cruising flight, which necessitated modification of the aftercooler to act as a heater to prevent the charge temperature falling below 40ºC.

The Merlin powered version of the DC-4, the Canadair Northstar was considerably faster than its radial engined equivalent. Noise was a problem initially due to the stub exhausts; the big radials tended to have collector rings; a crossover exhaust for the Merlin mitigated this to some extent. It wasn't an unsuccessful machine, but it wasn't ever going to capture the US market because it wasn't American.

As for perspective, how many DB powered airliners were there post WWII?

The Merlin wasn't successful as an airliner engine for many reasons - it hadn't be designed for that sort of duty for a start. It did rather better than the V-1710 though.

But perhaps the main reason for its "failure" as an airliner was that there just weren't suitable British airliners to bolt it onto. Lancastrian, York & Tudor could hardly compete with contemporary products from Lockheed & Douglas, because Britain had basically stopped airliner development in 1939 whilst the Americans had continued throughout the War (because they needed long-range transports anyway). They weren't about to put British engines onto their aeroplanes if they could possibly help it, so the considerable technical lead of the American airframers translated directly into market share for their engine manufacturers.

It's probably better to compare the Merlin's civil record with that of Hercules & Centaurus, which faced a similar airframe problem (though of course at this time Bristol had an aeroplane division as well, which provided them with a captive market for their engines).

In this context, the Merlin doesn't look so bad.

[Viper2000]

Originally Posted by Crumpp

The Allies did not have any direct injection engine technology to use.....

They could not make direct injection workable or practical using their fuel metering technology. Bosch's design, up until recently was the pinnacle of direct injection technology. It requires very high fuel pressures and the German system used a high pressure pump for each cylinder.

As already pointed out, post war, the turbine was supreme so why would any nation waste resources for a post war piston engine aircraft?????

Post WWII, it was perfectly within the Allies' rights to confiscate any and all German IP that they were interested in. So they could have made Bosch fuel injectors and put them into their engines at no great cost. They chose not to.

As for post-war engine development, the US government funded considerable development work on the R-3350 turbocompound, and indeed also upon the R-4360, both of which found their way into airline service.

Britain funded development of the Napier Nomad, which was a more ambitious take on the turbocompound idea (I strongly suspect that this engine was cancelled due to failure to meet its quoted performance; I modelled it in considerable detail a couple of years ago, and I could never make the quoted component efficiencies add up to the quoted SFC...).

Direct injection makes a lot of sense for naturally aspirated engines, compression ignition engines, or engines which operate over a wide power range. It's less attractive for a big aero-engine because if you're operating at fixed power with a reasonable amount of supercharge you should be able to attain excellent mixture distribution, and so the pragmatic solution is to have single point injection into the eye of the supercharger - which is basically what everybody ended up doing.

Of course, these days people aren't designing big piston aero-engines anymore, and they aren't supercharging*, so DI makes sense.

*and turbochargers tend to be bought from turbocharger companies, which means that injection into the eye of the turbo-supercharger impeller isn't really an option because it would be too much of a nightmare to organise the development effort - who pays for what etc?

[Viper2000]

Originally Posted by Crumpp

http://www.automobilemag.com/feature...ion/index.html

Single point injection has no advantages over direct fuel injection at all. The Supercharger is on a completely separate circuit and the engine still receives all the benefits of supercharging with the additional benefits of direct injection.

The supercharger is driven by the engine.

If you reduce the power consumed by the supercharger then you increase the brake horsepower and reduce the SFC.

Supercharger power consumption is just W*Cp*deltaT, ie W*deltaH.

Supercharger isentropic efficiency is

deltaH[actual]/deltaH[isentropic]

In the case of the Merlin, this figure was about 70%.

For isentropic, adiabatic compression,

T2 = T1(P2/P1)^(gamma/(gamma-1))

Hence it's trivial to calculate the isentropic deltaT, and deltaH.

DeltaT and deltaH both get smaller if we reduce T1.

Injecting fuel upstream of the supercharger reduces the temperature by about 25 K due to the latent heat of evaporation of the fuel.

This reduces the temperature rise across the supercharger, which is equivalent to increasing its adiabatic efficiency.

Clearly this confers an advantage to engines which inject fuel upstream of the supercharger. Given the considerable difficulty associated with increasing the aerodynamic efficiency of compressors, this advantage is not insignificant.

Mixture distribution is going to be very good provided that the charge temperature is sufficiently high for complete evaporation to be ensured. This will basically always be the case at high powers because deltaT is 100 K or more; indeed intercooling & aftercooling start to become necessary once you've got a lot of supercharge.

These advantages vanish at low non-dimensional power settings. Cars spend most of their time at very low non-dimensional power settings, and therefore DI wins hands down most of the time, especially if you go for CI, in which case it's almost no-contest.

In the end, the nature of all engineering trade studies is that the devil is in the detail. The optimum is a strong function of engine size and duty cycle, and we just don't build the sort of highly supercharged, high power spark ignition engines for which single point injection is attractive these days.

To use an analogy, old amplifiers used valves and therefore tended to have large transformers & rectifiers to produce the high DC voltages which allowed them to function. Most modern amplifiers are solid state, and they don't need those high voltages.

This doesn't mean that high DC voltages aren't still a good idea for valve amplifiers; I've got a pair of hundred watt half stacks sat next to me which run in excess of 400 V DC and sound great. But probably 99% of modern amplifiers for domestic use are solid state and so if you just ask "are high voltages a good idea for amplifiers" then the short answer is "probably not".

[CaptainDoggles]

Originally Posted by Viper2000

As for perspective, how many DB powered airliners were there post WWII?

I suspect there is politics involved with that.

[Viper2000]

Quite.

To a lesser degree the same argument applies to British engines, given that the most successful airliner airframes were American in 1945. Therefore comparison between the Merlin and the R-2800, R-3350 or R-4360 in the civil market isn't really fair; it makes more sense to compare it with the Bristol Hercules or Centaurus, and if you perform that comparison then the Merlin doesn't look quite so much of a "failure" in the civil market anymore...

[TomcatViP]

Originally Posted by Viper2000

... so the pragmatic solution is to have single point injection into the eye of the supercharger ...

I like that ... Good point.

However this shld lead to a certain amount of "latency" with DI eng being more reactive upon power changes by the pilot

Regarding the Merlin as an airliner eng, it has proved unreliable as high power value were run only at high boost and then prove to be non-efficient (the cruise power has always been low); Add to the disastrous engineering of Britain's airliner projects tht seems to hve been hand-ended by gvrnmt officials (don't take me wrong France had to face the very same situation until legitimate firms could emerge out of the bundle in earlies 60's) and you'll end with a more pragmatic vision of the failure of the British industry in perspective of the US success stories like Boeing/Doug/Lockheed right after war end.

Humm hve we run OT (out of topic) again?

[Crumpp]

I suspect there is politics involved with that.

It was....

Both Daimler-Benz and BMW were forbidden from even being in the aviation market.

Post-war, both companies withdrew from anything to do with aviation and produced automobile engines instead. Both are industry leaders from the moment they entered the market and that leadership continues today.

They produced some of the best engines in the world.

Injecting fuel upstream of the supercharger reduces the temperature by about 25 K due to the latent heat of evaporation of the fuel.

And injecting fuel directly into the combustion chamber is even better, Viper. How hard is that to understand?

If you reduce the power consumed by the supercharger then you increase the brake horsepower and reduce the SFC.

And that it is much more efficient to realize the power gains by directly injecting fuel into the combustion chamber than it is by dumping it into an intake manifold......

It's less attractive for a big aero-engine because if you're operating at fixed power with a reasonable amount of supercharge you should be able to attain excellent mixture distribution, and so the pragmatic solution is to have single point injection into the eye of the supercharger - which is basically what everybody ended up doing.

No, it is attractive and if we had the technology to do it on a cost effective basis, we would have done it. It is the ultimate fuel metering method for a piston engine in terms of power and efficiency. A single point injection simply cannot maintain a stoichiometric mixture in all the cylinders. That is why the EGT and CHT will always be different in each cylinder unless you have direct fuel injection.

[Crumpp]

In fact in the 1950's, we started doing it.....

In the R-4360C Wasp Major power-plant with CH 9 turbo-blower.....

The most important differences between the BH 4 installation (Fig. 3)
and the new power-plant (Fig. 4) are the elimination of the
intergral mechanically-driven supercharger and the carburettor
in the former in favour of the direct fuel injection and
no secondary (internal) supercharger in the latter.

http://www.flightglobal.com/pdfarchi...0-%200248.html