Topic: Rotary-Vtwin-Inline 4's and torque vs. HP

[Lumbee]

...I was having a conversation with a guy at work, and he mentioned he saw a picture of a bike with an airplane engine in it.  I think it might have been the one Jesse James build a few years back.  He asked me what the characteristics of a radial engine were.  I immediately said HP.  I then extrapolated that the engine must have low torque.  But then another co-worker jumps in and says that airplane radial engines are high HP and high torque.  I was think more cyclinders meant more HP, but less troque.  Wereas fewer cylinders mean more torque and less HP.  In general I think of twins (v or parallel) as having generally more torque and less HP.  Wereas inline 4's (and radial engines) have more HP and less torque.  Where is my thinking flawed here?

[Cvillechopper]

I always thought torque was more a function of the power potential and ability to quickly reach potential of each individual cylinder, regardless of the number of cylinders.  i.e. big cylinder, big intake/ exhaust ports= ability to make big boom immediately = torque.
smaller multiple cylinder engines get HP in the high end by lots of throughput and ability to rev much higher than larger jugs.  Less surface area between cylinder walls and pistons = less friction = less potential for kabloomy at high rpm, etc.

Not very technical, I know but it's the way my grandad explained it to me years ago and I haven't found something to help explain it better yet.

[nickjtc]

Not being an expert I'll just throw in a thought. In an aviation application power delivery has to be consistent at a constant engine speed....lots of power to get the thing off the ground, then enough power to 'cruise' economically.

So the thought that more cylinders = more power makes sense. Torque is perhaps not an issue since the engine is only turning a propellor?

The radial engine concept was developed as a way of getting more cylinders, but at the same time not compromising cooling. These engines first came on the scene in the days before liquid cooling.

[mlinder]

Radial and rotary are different. I can go into that if you want, but it isn't important for this conversation.

Number of cylinders really doesn't matter on the torque front.
Remember that HP is torque.

(Torque x Engine speed) / 5,252 = Horsepower is the equation for conversion. Look at a dino. The HP and Torque curve always cross at 5252 rpm.

What makes torque is the characterisitic of the engine.
The reason it seems like multiple cylinder engines seem better for more 'HP' and less 'TQ' is because they are designed to rev higher in a lot of cases. This is a design choice, though, and not an inherent characteristic of multi cylinder engines. Look at the equation, "tq * engine speed". The things that allow an engine to rev higher are generally poor for low end power (notably, no back pressure and more importantly, very oversquare piston design).

However, even with a lower torque number, higher revs = more hp (again, torque x RPM).

To put it simply, the design of the engine itself is the deciding factor of low rpm high tq vs. high rpm high hp, not the number of cylinders.

[nickjtc]

Radial and rotary are different. I can go into that if you want, but it isn't important for this conversation.

I could never understand the rationale behind a rotary engine. The concept of making the whole engine spin, with the crankshaft staying still, just seems so counter intuitive from an engineering standpoint. There must have been a good reason for it to be done though. Cooling? Better manouverability if you are turning with the torque reaction of the engine?

[mlinder]

Radial and rotary are different. I can go into that if you want, but it isn't important for this conversation.

I could never understand the rationale behind a rotary engine. The concept of making the whole engine spin, with the crankshaft staying still, just seems so counter intuitive from an engineering standpoint. There must have been a good reason for it to be done though. Cooling? Better manouverability if you are turning with the torque reaction of the engine?

Nope.
Lack of friction and reciprocating mass, as well as overall engine size for similar displacement.
There is substantially less surface-friction between parts, as very little of the surfaces touch each other. Just the apex of the rotor to the seals of the rotor housing.
Also, since the rotor is turning in a small, tight circle inside the housing, the rotor never has to change directions like a piston. It only has to make a 1 degree variation in direction 360 times per rotation.
These two things also allow much higher revving.
The engine is also considerably smaller for total combustion chamber size than a traditional piston engine, saving weight and space.
To top it all off, there are FAR fewer moving parts.

[Seyser]

Radial and rotary are different. I can go into that if you want, but it isn't important for this conversation.

I could never understand the rationale behind a rotary engine. The concept of making the whole engine spin, with the crankshaft staying still, just seems so counter intuitive from an engineering standpoint. There must have been a good reason for it to be done though. Cooling? Better manouverability if you are turning with the torque reaction of the engine?

Its definately not cooling, Rotary engines generally run very hot. The rotary's primary strength is its excellent power to weight ratio. The rotary also runs very smoothly, This is due in part to its famed rotary motion instead of the back and fourth motion of pistons.

[kslrr]

To add my 2 cents based on my knowledge of railroad engines.  A typical Diesel Electric engine of the '80's and '90's was a 751/16.  That means 751 Cubic Inches per cylinder with 16 cylinders (a piston con rod assembly is 6 feet tall).  The engine idles at 100 rpm and makes peak HP (4,000) @ 900 rpm.  Torque is never mentioned but calculates to be 23,342 ft/lbs!  Todays engines make 6,000 HP, though I am not sure at what RPM.

These engines make gobs of torque because they have very long strokes.  The size of the cylinder doesn't hurt either.

To look at the definition another way, Torque is a measure of work.  HP is a measure of work performed over time.  The time component is Rotations Per Minute.

[SteveD CB500F]

My 2p.

In the last two years I have owned a VFR800 and a Triumph 955i triple (and two 500/4s, and a 550K3, and a Bandit...)

The VFR had 4 x 200cc and delivered 110bhp at 12,500 rpm
The Triumph has 3 x 318cc and delivers 108bhp at 9,250rpm

Guess which is easier to ride in town ??

The Trumpet will pull 5th and 6th gear from 2500 rpm, the Honda, bugger all under 3500 rpm

[nickjtc]

OK, I am really showing my age here, and I went off down my own little side alley in this discussion.

My first thought when rotary engines were mentioned in this context is the engine configuration that looks like a radial engine, but in which the crankshaft is fixed and the whole of the engine (crankcase, cylinders etc) rotates around it. The propellor is bolted to the engine.

http://en.wikipedia.org/wiki/Rotary_engine

Several notable aircraft of the First world War used this configuration, notably the Sopwith Camel.

Yes, of course a modern rotary (aka Wankel) engine is a completely different kettle of fish!

[Rocking-M]

Just curious, since the bmw engine is essentially a radial engine minus the
other cylinders, did it originally rotate around it's crank?

The engine with two cylinder makes good low end torque, so I would
image if it had the rest of it's cylinders it would make even better torque

[mlinder]

Just curious, since the bmw engine is essentially a radial engine minus the
other cylinders, did it originally rotate around it's crank?

The engine with two cylinder makes good low end torque, so I would
image if it had the rest of it's cylinders it would make even better torque

No, it didn't, so far as I know. It rotated the crankshaft whilst the cylinders were stationary.
The reason they made the earlier radial engines rotate around a staionary shaft while turning the propeller was for cooling.
And yes, if it had it's other cylinders, it would make that much more torque, minus frictional losses.

By the way, WW1 BMW engines were in-line. The made radial engines for WWII. They'd gone to water cooled by then, no need for rotating engines.

[ic455]

Radial and rotary are different. I can go into that if you want, but it isn't important for this conversation.

I could never understand the rationale behind a rotary engine. The concept of making the whole engine spin, with the crankshaft staying still, just seems so counter intuitive from an engineering standpoint. There must have been a good reason for it to be done though. Cooling? Better manouverability if you are turning with the torque reaction of the engine?

Its definately not cooling, Rotary engines generally run very hot. The rotary's primary strength is its excellent power to weight ratio. The rotary also runs very smoothly, This is due in part to its famed rotary motion instead of the back and fourth motion of pistons.

not to mention a redline over 10k rpm

[Gordon]

Wow!  I've never heard of this bike before, but there was a link to it on the Wikipedia page that Nick posted.  A rotary-engined motorcycle where the engine served dual purpose as the front wheel!  

http://en.wikipedia.org/wiki/Megola

[jabbadeznuts]

I thought torques was more closely related to the engine's stroke than overall displacement. 

[mlinder]

I thought torques was more closely related to the engine's stroke than overall displacement. 

No...

[GroovieGhoulie]

I also heard that the early radial engines rotated around a fixed crank for more complete atomization of the fuel-air mixture.  At the time carburetors were a new technology and a bit of a black art, (along with being VERY crude), so designers found that spinning the whole works better mixed the fuel and air and made the engines run better with the carb technologies for the day.

[kslrr]

I had hear it was a solution to the problem of flying upside down and loosing oil pressure.  With the engine spinning, there is no up nor down for the oil.

[jabbadeznuts]

I thought torques was more closely related to the engine's stroke than overall displacement. 

No...

Well, since a longer stroke means that the crank is further from "center" doesn't that mean that there's more torque?

[SteveD CB500F]

Certainly more inertia.