Engine Displacement?

[Charge]

I have wondered what effect does engine displacement have in aircraft?

The engine of the fighter airplane usually runs in maximum or near maximum allowable RPM. So, does the the extra displacement give any bonus when compared to engine giving nearly the same power with smaller displacement? e.g. More time running at maximum engine output?

If we compare e.g. Spit1 and 109E. Spit has 1066BHP and 38ltr and 109E has 1100BHP and 42ltr. (I wonder if  I got this right??)

Does the displacement difference give the 109 the ability to pull steeper climb when compared to Spit, which is a bit heavier and has smaller engine displacement (but better wingloading) because the bigger displacement gives it more torque which helps it maintain more optimal RPM in difficult situation?

-Charge+

[Tony Williams]

The Merlin's capacity was only 27 litres (I can't recall the DB's offhand, but it was certainly bigger).

My understanding is that the Merlin matched the performance of the bigger German engines because it ran at much higher supercharger boost pressures, probably due to the availability of higher-octane fuel.

Tony Williams: Military gun and ammunition website and discussion
forum

[Maverick]

Higher displacement  would equal more HP, all other factors being equal. The more cubic inches (or litres) means more ability to pump out power. This means more power to benefit performace in speed and climb.

Adding things like a supercharger to benefit high alt performance or to increase manifold pressure over a "normal" limit such as WEP would also allow the engine to put out more horses to increase plane speed and climb.

[joeblogs]

It's kind of an apples and oranges comparison.  As people have already mentioned, the Merlin developed a lot of HP out of 27 liters.  To do it, it had to tolerate high RPMs, a very high boost, and it required higher octane fuel too.

The DB601 displaces something like 34 liters, but does not take the same amount of boost as does the Merlin, nor quite as high RPM.  Instead, it has a higher compression ratio (the ratio of the volume at the top of the cylinder with the piston at the bottom of its stroke relative to the volume that remains with the piston at the top of its stroke).  Higher compression yields more power and somewhat better fuel efficiency.

Boost vs compression is complicated.  Typically supercharger efficiency declines as you design one to generate more boost.  But RR developed the most efficient gear driven superchargers and intake manifolds of that time.  

On the other hand, a higher compression requires a physically stronger engine, and typically implies more weight too.  That is because the break-mean-effective pressure (think average pressure) of the DB601 is lower than the Merlin, but it's peak pressure is higher than the Merlin.  But if the strength is there, the engine will be more durable under ordinary conditions.

I have some charts on all this I'll try to post.

Originally posted by Charge
I have wondered what effect does engine displacement have in aircraft?

The engine of the fighter airplane usually runs in maximum or near maximum allowable RPM. So, does the the extra displacement give any bonus when compared to engine giving nearly the same power with smaller displacement? e.g. More time running at maximum engine output?

If we compare e.g. Spit1 and 109E. Spit has 1066BHP and 38ltr and 109E has 1100BHP and 42ltr. (I wonder if  I got this right??)

Does the displacement difference give the 109 the ability to pull steeper climb when compared to Spit, which is a bit heavier and has smaller engine displacement (but better wingloading) because the bigger displacement gives it more torque which helps it maintain more optimal RPM in difficult situation?

-Charge+

[funkedup]

Weight and size increase pretty much linearly with displacement.  Fuel consumption goes up too.

Variation in power with RPM is not a big factor for WW2 fighters, because advanced types all had constant-speed props.

[MiloMorai]

DB601A
displacement = 33.9 L(2069ci).

max rpm (109E)

level flight = 2400, later 2700
dive = 3000
CR = 6.7:1

Merlin II

CR = 6.0:1
max rpm = 3000

DB603

44.5 L(2715ci)
2700 rpm
different CR in each cylinder bank

DB605

35.7 L(2178ci)

[joeblogs]

You are confusing propeller RPM and engine RPM.  

All that a constant speed propeller does is translate the engine RPM into the most efficient pitch of the propeller.  This will affect engine RPM simply because it places a load on the engine.  But that does not imply that engine RPM is irrelevant.

A piston engine is a complicated blow dryer.  The more air pushed through it (and heated) the more horsepower.  One way to push more air through it is to run it at higher RPM.  Unfortunately forces on the engine increase dramatically with more RPM so it takes a very well designed (or small) engine to develop very high RPM.

Aces is modeled with constant speed props.  Experiment with  RPM and you will see it affects (1) your power and (2) your fuel consumption.

-Blogs

Originally posted by funkedup
Weight and size increase pretty much linearly with displacement.  Fuel consumption goes up too.

Variation in power with RPM is not a big factor for WW2 fighters, because advanced types all had constant-speed props.

[pugg666]

Aces is modeled with constant speed props. Experiment with RPM and you will see it affects (1) your power and (2) your fuel consumption.

it should but it does not have an effect on fuel consumption in AH. It has been tested by numerous people already. The only use in AH of reducing prop rpm is to improve your glide ratio.

[funkedup]

Blogs what I meant was this:

A typical scenario for automobiles is where you have a large displacement engine and a small displacement engine (imagine 2 liter I-4 and a 5 liter V-8).  They both might make the same peak horsepower if the I-4 revs high enough, but the V-8 will have a flatter power curve and still might win a drag race.

In a WW2 fighter it was not a big deal to have a peaky power curve because the CSVP prop could hold engine speed near the RPM for peak power.

[funkedup]

Originally posted by pugg666
it should but it does not have an effect on fuel consumption in AH. It has been tested by numerous people already. The only use in AH of reducing prop rpm is to improve your glide ratio.

It depends on the airplane.  It definitely works for the Mustangs.  It doesn't perfectly match the book figures for gal/hr at various rpm/mp settings, but it's reasonably close.

[joeblogs]

I does work for the FW190 in Aces.  I'm surprised it doesn't work for all planes.

-Blogs

Originally posted by funkedup
It depends on the airplane.  It definitely works for the Mustangs.  It doesn't perfectly match the book figures for gal/hr at various rpm/mp settings, but it's reasonably close.

[funkedup]

My finding was that reducing RPM always reduced fuel flow rate, but it didn't always increase specific range.

To fly at a given speed, you would get better specific range at maximum RPM than at any other RPM.  Except on the Mustangs.
 
BUT

It's been a long time since I've tested it, so maybe it works on all of them now.  I don't think it was an error by HTC, just a choice not to spend time implementing a feature that 99% of the user base won't even notice.

[flakbait]

DB-605A
Displacement: 2179 cu-in (35.7L)
Compression ratio: 7.3:1 port cylinders, 7.5:1 starboard cylinders
Bore, stroke: 6.1x6.3 in

P&W R-2800
Displacement: 2800 cu-in (45.9L)
Compression ratio: 6.65:1
Bore, stroke: 5.8x6 in

Packard V-1650-1
Displacement: 1649 cu-in
Compression ratio: 6:1
Bore, stroke: 5.4x6 in

Alison V-1710F
Displacement: 1710 cu-in
Compression ratio: 6.65:1
Bore, stroke: 5.5x6 in


Specs lifted from:
http://www.eaa1000.av.org/technicl/engemp/engemp4.htm
http://www.aviation-history.com/index-engine.htm


-----------------------
Flakbait [Delta6]
Delta Six's Flight School
Put the P-61B in Aces High

[Charge]

Funked got exactly what I was wondering about. The larger engine should have it easier to reach max RPM due to its bigger torque and hold it there easier in difficult loading situations. Going up-hill would propably be a proper equivalent.

The difference is propably not so big in aircraft, however, as the power transition from engine to substance in effect (ground/air) is different. (e.g. in aircraft the engine still has power to rev but the wing has already lost the lift, stalling, causing the propeller to just cavitate through air.)

I guess that in this comparison it doesn't really affect anything whether the engine has a bigger displacement or works on higher boost as long as the output power is fairly similar? The high-boost engine is just more expensive and more difficult to manufacture and requires more maintenance.

-Charge+

[Golfer]

you make a good point about higher rpms and smaller engines.  take for example (apples to oranges to a Merlin and a Benz) the 2 airplanes i am most familiar with...

Cessna 172
Cessna 150 (looking at one to buy to fatten up the ol logbook)

RPMs i use as a current renter (not worried about saving fuel since i rent them Wet anyway)
C-172 = Lycoming O-320, cruise rpm 2400 leaned
C-150 = Continental O-200, Cruise rpm 2550 leaned (important in these little continentals)

Just making a point that higher rpms and size of engine go hand in hand.  Same with RC engines, you can have some small small small Norvel's turning 20,000 rpms whereas a larger engine might spin 12-13 grand.

That's all