Differences between automotive & airplane engines

On Fri, 10 Feb 2006 13:34:18 +0100, Philippe Vessaire
wrote:

Chris Wells wrote:


How are "normal" airplane engines tuned to run at a lower rpm? What
changes would have to be made to an automotive engine to shift the
power band down accordingly?

the airplane enemy is weight.

Any engine may fit, the lighter is the better.

An automotive engine, with PSRU is always heavier than an airplane one.
An automotive engine burn the same amount of gas than an airplane one

Is automotive engines cheaper than a 2000h core of airplane engine? (with
the PSRU).

By
Not ALWAYS heavier, but usually. How much depends on a lot of
factors. An automotive engine without a psru can be very close. It can
also burn less fuel, under some circumstances.

An O200 and a Corvair weigh virtually the same (within 30 lbs), with
electrical systems, and provide virtually the same hp and thrust.

Cheaper? most definitely can be - and certainly is cheaper to overhaul
when the time comes. And the automotive engine MAY run longer between
major overhauls.
I can build a zero timed Corvair for not much more than the cost of
rebuilding one cyl on a Lycoming.

On 10 Feb 2006 05:55:48 -0800, "Lou" wrote:

I'll agree with the automotive engine with PSRU being heavier, but are
you sure about your other statement "the lighter the better"?
I'm currently looking at an engine that is 100lbs lighter than the one
recommended for my plane. Although cutting 100lbs from the total
weight is a dream come true, it brings up the question of weight and
balance. I can move the engine forward to make up the difference in
balance, but I don't know how far or how to find out.
Lou
That is simple to determine. Get a good book on aircraft design and do
the math. multiply the weight times the distance in inches from the
genter of gravity of the plane to the center of mass on the original
engine. Then devide that number by the weight of the new
powerplant.The answer is the distance in inches to the center of mass
of the engine.

On Fri, 10 Feb 2006 12:46:28 -0500, "Peter Dohm"
wrote:

wrote in message
roups.com...

Lou wrote:
I'll agree with the automotive engine with PSRU being heavier, but are
you sure about your other statement "the lighter the better"?
I'm currently looking at an engine that is 100lbs lighter than the one
recommended for my plane. Although cutting 100lbs from the total
weight is a dream come true, it brings up the question of weight and
balance. I can move the engine forward to make up the difference in
balance, but I don't know how far or how to find out.
Lou

You weigh the airplane without the engine installed and
calculate a balance point for it. Knowing the weight of the engine, you
then figure the arm at which it needs to be located to bring the
airplane's empty CG to the point the designer calls for it. Not a big
deal at all. Pages 134 and 135 of William Kerschner's Advanced Pilot's
Flight Manual shows how.

Dan

However, you will also be changing the area and arm relationships of the
side view of the aircraft (there is a name for this which I cannot recall)
and the size of the verticall fin will need to be increased if you are to
retain the same yaw stability. Then, because of the increased area of the
vertical stabilizer, a larger rudder would be needed to retain the original
crosswind landing capability. In addition, due to the increased planform
area forward of the CG, a larger horizontal stabilizer may well be required
to prevent any sort of deep stall or flat spin tendency. Finally, just as a
larger vertival stabilizer requires a larger rudder, a larger horizontal
stabilizer will very likely require a larger elevator.

To put it another way: Engineering is the science of compromise, and an
airplane is a series of compromises flying in close formation.

Peter

You are correct - but 4-6 inches on a 20 foot plane does not make a
significant difference in the yaw and required rudder size. And 4 to 6
inches can correct for a fair amount of weight.

On Fri, 10 Feb 2006 19:10:21 GMT, ORVAL FAIRAIRN
wrote:

In article ,
Chris Wells wrote:

How are "normal" airplane engines tuned to run at a lower rpm? What
changes would have to be made to an automotive engine to shift the
power band down accordingly?

You are entering an engineering thicket when you decide to convert
automobile engines to aeronautical use.

One item nobody has yet mentioned is the matter of thrust bearings. An
automobile engine is designed to deliver its power through a clutch, to
a gearbox, with relatively low axial forces imparted to the crankshaft.

A direct-drive aircraft engine, however, delivers its power to a
propeller, which pulls (or pushes) axially on the crankshaft. If you
took an automobile engine and hung a prop on the end of the crank, you
amy or may not have enough thrust bearing to take the loads.

A properly-designed PSRU will have a thrust bearing to take those loads.
Some PSRUs, however, may impart side loads to the power end of the crank
and result in wear and fatigue issues.

There is an axiom for homebuilders: If you want to develop engines,
convert automobile engines; if you wish to fly, use aircraft engines.
The thrust bearing area on a corvair is almost the same as on a 100HP
Lycoming. It's at the wrong end of the crank, but??

On Fri, 10 Feb 2006 09:24:41 -0800, "Al"
wrote:


"Chris Wells" wrote in message
.. .

How are "normal" airplane engines tuned to run at a lower rpm? What
changes would have to be made to an automotive engine to shift the
power band down accordingly?

Lengthen the stroke. High RPM engines have a large bore, and short stroke.
Low RPM engines have a longer stroke, and smaller bore, all else remaining
equal.

Al

Not necessarily so. The myth that high speed engines must be over
square, and low speed engines under square is just that. Piston speed
is what matters, to a large extent, and rod angle - which dictates rod
length.

When you say all else remaining equal, you leave yourself wide open,
because it seldom is.

On Fri, 10 Feb 2006 19:43:24 +0000, Chris Wells
wrote:


I'm well aware of the purpose of the PSRU, I'd like to know if it's
feasible to convert an automobile (or other) engine to run at an RPM
low enough so that a PSRU wouldn't be necessary. I'm thinking a custom
camshaft would be needed, and different ignition timing, what else?


Valve timing is a good start. Particularly exhaust valve opening
timing. Intake and exhaust tuning also changes - longer and smaller
runners for lower speeds. Look at the short intake runners on a Rotax
912, compared to the long intakes on a Lyco. Smaller runners promote
higher intake and exhaust velocities on slower engines, causing better
filling and purging of the cyls.

Anything you can do to pack more air into the cyl, and to get it ( the
products of combustion ) out will provide more power.
The more air an engine consumes in a minute, the more power it
produces. Turning it faster consumes more air. Supercharging consumes
more air, power tuning the intake and exhaust consumes more air (at
the same, proper, designed speed)

Then make sure the engine can get rid of the heat produced by
consuming all that air. This can be the limitting factor, as on the VW
engine.

clare at snyder.on.ca wrote in message
...
On Fri, 10 Feb 2006 12:46:28 -0500, "Peter Dohm"
wrote:

wrote in message
roups.com...

Lou wrote:
I'll agree with the automotive engine with PSRU being heavier, but
are
you sure about your other statement "the lighter the better"?
I'm currently looking at an engine that is 100lbs lighter than the
one
recommended for my plane. Although cutting 100lbs from the total
weight is a dream come true, it brings up the question of weight and
balance. I can move the engine forward to make up the difference in
balance, but I don't know how far or how to find out.
Lou

You weigh the airplane without the engine installed and
calculate a balance point for it. Knowing the weight of the engine, you
then figure the arm at which it needs to be located to bring the
airplane's empty CG to the point the designer calls for it. Not a big
deal at all. Pages 134 and 135 of William Kerschner's Advanced Pilot's
Flight Manual shows how.

Dan

However, you will also be changing the area and arm relationships of the
side view of the aircraft (there is a name for this which I cannot
recall)
and the size of the verticall fin will need to be increased if you are to
retain the same yaw stability. Then, because of the increased area of
the
vertical stabilizer, a larger rudder would be needed to retain the
original
crosswind landing capability. In addition, due to the increased planform
area forward of the CG, a larger horizontal stabilizer may well be
required
to prevent any sort of deep stall or flat spin tendency. Finally, just
as a
larger vertival stabilizer requires a larger rudder, a larger horizontal
stabilizer will very likely require a larger elevator.

To put it another way: Engineering is the science of compromise, and an
airplane is a series of compromises flying in close formation.

Peter

You are correct - but 4-6 inches on a 20 foot plane does not make a
significant difference in the yaw and required rudder size. And 4 to 6
inches can correct for a fair amount of weight.

You are right. I should have at least asked the weight of the original
engine.

clare at snyder.on.ca wrote in message
...
On Fri, 10 Feb 2006 16:28:10 GMT, Alan Baker
wrote:

In article .com,
wrote:

But when we DO use 'horsepower' we must be careful to
never use it in isolation, always identifing the rotational speed at
which that 'horsepower' is being produced.


Absolutely and utterly wrong.

It is *torque* which must always be associated with the rotational
speed
at which it is being produced.

Read that first sentence again. He's not wrong; he just
didn't specify "torque" for those who don't know the relationship
between it and RPM and HP.
When you say "absolutely and utterly" it should be used
only where it applies. Clearly, that's not here.

But that's my point. He is absolutely and utterly wrong, when he says
that you need to know the rotational speed before you know all you need
to know when you know the horsepower.

With horsepower, you can use gearing to get any rotational speed you
want; the horsepower remains constant. Torque changes with gearing.

Yes, you CAN use gearing, at the expense of complexity.And efficiency.
Much better to design the engine to produce the power you need at the
speed you need it. However, sometimes you trade efficiency and
durability for weight - and a geared 1.2 liter 80 hp engine running
at 6000 RPM can weigh significantly less than a direct drive 2.7 liter
engine providing the same power at 2800 rpm. (well, about 40 lbs less,
anyway)

Ya' know ... there is a real problem with this entire discussion. Not just
this latest thread, but the discussion in general, and I really feel a need
to mention it before I turn in for the night--which is another ting that I
fell a need to do.

The problem, as I see it, is that there may be nearly as much difference
between different kinds of airplanes as there is between the different kinds
of ground vehicles that can be operated on public roads. That's just
counting airplanes, not helicopters, etc...

And we can probably all agree that a faster airplane can efficiently use a
smaller, and faster turning, prop for its horsepower than can a slower
airplane.

Some of us are mostly interested in airplanes that really need a redrive to
get good propeller efficiency from a 40 HP VW. Others are interested in
slippery airplanes that cruise at 150 to 200 kts. My interest is in the
faster type of airplane, and the only reason the specification isn't for
something even faster is a desire to keep the simplicity of a fixed pitch
prop. Therefore, if I want to use the old formula of 0.2G static thrust for
good takeoff performance on a 150 kt airplane, I only need to divide the
expected gross weight of the airplane by 10 to arrive at a reasonable
horsepower figure. (Since I want a static thrust of one fifth of the gross
weight, and also since each horsepower results in 2 pounds of thrust at the
150 kt speed--or would if efficiency was 100%) I really DON'T care about
efficiency, because I only intend to operate at low speed and high power for
less than a minute per flight. Propeller efficiency will always be zero, by
mathematical definition, at the beginning of the take off roll; and my
numbers work just fine with 40% efficiency during the initial climb to clear
the obstacles. On the other hand, if your plan is to cruise at 60 kts, with
a proportionately slower initial climb speed, then you probably need a
larger diameter prop than I do, even with a much lighter and less powerful
airplane.

We really need to look at what is workable, reliable, and affordable for
each specific application. I admit to being a long time advocate of
automotive conversions, and the various GM and D-C all aluminum 60 degree
V6s from 3.0 to 3.7 liters really do look promising; but I really would have
to think long and hard before I trying to adapt one to an airplane that has
already been designed around a standard airplane engine. Just making the
cooling system work reliably, with reasonable drag, would probably cause
insomnia!

Peter

In article ,
Sparkle wrote:

clare at snyder.on.ca wrote:
Alan Baker wrote:

wrote: ...
But when we DO use 'horsepower' we must be careful to never use
it in isolation, always identifing the rotational speed at which
that 'horsepower' is being produced.

Absolutely and utterly wrong.

It is *torque* which must always be associated with the rotational
speed at which it is being produced.

You are both arguing the same thing, since horsepower is the product

They're saying two very different things. Alan is correct.

All the torque in the world won't move anything if it is not allowed
to cause motion, or speed.

That's why Alan said torque must always be associated with rotational speed.

In the context of engines turning airplane propellers (or any other
attempt to do what physicists call "work,") I'll agree that it's
meaningless to talk about torque without talking about RPM. But, since
horsepower varies with RPM, isn't it also meaningless to talk about
horsepower without talking about RPM?

It seems to me that Mr. Hoover is correct, and so is Mr. Baker, with the
exception of his comment about Mr. Hoover being wrong.

In article ,
clare at snyder.on.ca wrote:

On Fri, 10 Feb 2006 06:12:41 GMT, Alan Baker
wrote:

In article .com,
wrote:

A by-product of that lack of education is how Americans view
'horsepower,' typically insisting that 50hp (at 5000rpm) is EXACTLY THE
SAME as 50hp (at 1000rpm). Indeed, most will whip out their calculator
and 'prove' they are identical :-) But as the Wright brothers
discovered more than a hundred years ago, horsepower is not a factor in
the equation of flight. With powered flight, the factor we must
concern ourselves with most is thrust. Working back through the
equation, for a given propeller efficiency & rpm we will eventually
arrive at a given quanta of torque which then may be converted into
units of 'horsepower,' should we wish to do so, although it serves no
useful purpose. But when we DO use 'horsepower' we must be careful to
never use it in isolation, always identifing the rotational speed at
which that 'horsepower' is being produced.

Absolutely and utterly wrong.

It is *torque* which must always be associated with the rotational speed
at which it is being produced.

You are both arguing the same thing, since horsepower is the product
of torque and speed. With the commonly understood units of RPM and ft
lbs, the product needs to be devided by the constant 5252 to provide
horsepower.

I can say my engine produces 91 HP at 3000 RPM, or I can say it
produces 160 ft lbs torque at 3000 rpm, and I am saying exactly the
same thing. The fact the engine may also produce 140 hp at 5000 rpm is
totally immaterial except to indicate it MIGHT be able to stand up to
producing 90 hp for a significant amount of time without overheating.

All the torque in the world won't move anything if it is not allowed
to cause motion, or speed.

No. Incorrect.

You can simply say your engine produces 91 hp, and then you can use a
gearbox to produce any combination of torque and RPM whose product is
that HP.

Answer this question:

Would you prefer an engine that produces 2000 ft-lbs of torque and 20
hp, or one that makes 20 ft-lbs of torque and 2000 hp?

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."