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Author Topic: Engine Displacement?  (Read 1240 times)

Offline joeblogs

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Displacement vs torque
« Reply #15 on: August 14, 2003, 05:25:00 PM »
The problem there is inertia.  The bigger engine has longer and heavier conrods and pistons.  A well designed smaller engine will accelerate to its max power faster.

There is a similar problem with the supercharger.  Suddenly changing the amount of air sucked into the engine causes all sorts of transitory effects.  They are worse the larger the supercharger...

-Blogs

Quote
Originally posted by 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+
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Offline VO101_Isegrim

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Engine Displacement?
« Reply #16 on: August 15, 2003, 01:19:46 PM »
As far the Merlin and the DB601/605 series went, its a perfect example for the "There`s no replacement for displacement" rule.

Merlins achieved just about the same powers as DBs, but they had to use higher RPM, higher grade (and thus more expensive)  fuel,higher boosts to do that. Higher boost worked against the use of higher compression rate, which resulted in worser effiency, or in other words : Merlins consumed a lot more fuel than DBs for the same horsepower output, by about 1/3 more at high powers. The positive effects of larger displacement can be seen very well on the DBs: the larger, more powerful DB605 actually consumed less while producing more power than the DB601...

Also interesting to note, that in order to withstand the forces generated by the much higher boost rates, Merlins lost the possible edge they could have in size and displacement. Practically the weight of Merlin`s (27 liters)  and the larger displacement DB`s (33.4 and 35.7 liters IIRC) was the same, in fact, later Merlins were a bit heavier. Dimensions were the same  almost for milimeter. It seems the neccesary strenghtening of the engine block stole away the gains with a smaller displacement.

Rough empty dry weights were the following:

Single-Stage Merlins and DB601As : 610 kg
Two-Stage Merlins (61-66) : 741-749kg.
DB 605A: 740kg (approx.)
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Offline Orka

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Engine Displacement?
« Reply #17 on: August 20, 2003, 02:50:30 PM »
Not same subject but realy near.
Some days ago someone ask about diferencial CR in DB series( SimHQ) except 601A. Looking for some info just found ,and not too much :( , tha was a matter of air intake/supercharger position. But know i'v readed most problabel is about a slightly different head volume.
Any clue?

bsitos
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Offline SFRT - Frenchy

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Engine Displacement?
« Reply #18 on: August 20, 2003, 04:39:03 PM »
So the Merling was a rice rocket, and the DB605 a good old BigBlock?
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Offline MiloMorai

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Engine Displacement?
« Reply #19 on: August 20, 2003, 06:56:44 PM »
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Offline Orka

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Engine Displacement?
« Reply #20 on: August 20, 2003, 07:42:07 PM »
Quote
Originally posted by MiloMorai
Orka, there is a thread here, but seems to have died.

http://oldsite.simhq.com/simhq3/sims/boards/bbs/ultimatebb.php?ubb=get_topic;f=98;t=003080


I was in that thread as you can see, just looking for more oppinions here ;)
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Offline Charge

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Engine Displacement?
« Reply #21 on: August 21, 2003, 03:51:58 AM »
What is the direction of rotation in DB? Maybe the higher CR side has a better angle against the roof of the engine (inverted V) than the other side thus enabling it to run at its fulla CR whereas the other side has worse angle and has its CR dropped?

Looking the engine from behind: The crankshaft turns antiCW and the right side has higher CR?

I don't think the pressure in intake manifold can vary depending of the side of the supercharger as the the downpipe from SC to IntakeMF goes down in middle of the engine when looked from behind.

-Charge+
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Offline mw

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Engine Displacement?
« Reply #22 on: August 21, 2003, 07:56:34 AM »
Clarification on the Spit I:

Merlin III
1649 cu. in.
27.02 litres
Net weight dry: 1375
H.P/RPM/Altitude: 1310/3000/9,000'
Rated Boost: 12 lbs./sq.in.

Do you guys realize that your Spit I is modeled with performance using 87 octane/6.25 lbs/sq.in??????  That's short about 260 hp.  Hurri is short changed as well.
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Offline chisel1

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Engine Displacement?
« Reply #23 on: August 25, 2003, 01:31:52 AM »
The reason for the CR difference I would guess is probably a cooling system imbalance after the 6xx series block was bored out bigger?
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Offline MiloMorai

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Engine Displacement?
« Reply #24 on: August 25, 2003, 03:01:26 AM »
Quote
Originally posted by Charge
What is the direction of rotation in DB? Maybe the higher CR side has a better angle against the roof of the engine (inverted V) than the other side thus enabling it to run at its fulla CR whereas the other side has worse angle and has its CR dropped?


I think you should re-think this statement.;)
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Offline Charge

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Engine Displacement?
« Reply #25 on: August 25, 2003, 03:09:00 AM »
C'mon Milo, help me out here. Could it be something that has to do with force vectors inside the engine?

I mean the same reason why the conrod is not attached to the middle of the piston but slightly to the other side of it.

:confused:

-Charge+
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Offline Angus

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Engine Displacement?
« Reply #26 on: August 25, 2003, 09:44:18 AM »
Are you sure our Spit I is modelled with 87 octanes? Sheesh.....how nice it would be to have the 100 oct already used in th BoB......:D
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Offline GScholz

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Engine Displacement?
« Reply #27 on: August 25, 2003, 10:03:24 AM »
Achtung! Schpitfire lover!!! :D
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Offline mw

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Engine Displacement?
« Reply #28 on: August 25, 2003, 11:58:06 AM »
Angus, I'm certain.  The full throttle height and very slow SL speed are dead give-aways.  What you have modeled here is not representative of Spit I performance during the BoB.
« Last Edit: August 25, 2003, 12:23:28 PM by mw »
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Offline joeblogs

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87 octane
« Reply #29 on: August 26, 2003, 01:44:58 PM »
Why shouldn't it be modeled that way?  The Merlin I and II engines were designed and rated for that fuel and the UK didn't get 100 octane until US tankers showed up during the Battle of Britain.

-Blogs

Quote
Originally posted by mw
Clarification on the Spit I:

Merlin III
1649 cu. in.
27.02 litres
Net weight dry: 1375
H.P/RPM/Altitude: 1310/3000/9,000'
Rated Boost: 12 lbs./sq.in.

Do you guys realize that your Spit I is modeled with performance using 87 octane/6.25 lbs/sq.in??????  That's short about 260 hp.  Hurri is short changed as well.
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