18 September 2010

Engine Configuration, Torque, and Horsepower

You may recall, when I started searching for a motorcycle to put in my garage, I was interested mainly in a bike to use on my commute to and from work. Secondarily, I thought it might be nice if it was big enough and comfortable enough for Sara Jean to hop on and ride along every now and then. Because of an experience I had in the rain several years ago, I made the decision that any bike I buy will either be driven by a belt or a driveshaft. That's not so limiting today as it was a few years ago... most Harleys are belt driven, and most other major manufacturers build at least one shaft-driven bike.

The other consideration in this quest was engine configuration...
For me, bikes with one cylinder vibrate too much for long trips. Some two-cylinder machines vibrate badly, some are smoother. Bikes with three cylinders or more vibrate less, but then you start encountering more complexity and more costs when repairs or preventive maintenance are needed. I started studying different bikes and the options they offered.

My first real motorcycle was a Harley Davidson "Sprint", powered by a single-cylinder 250cc engine. As you can see in the photo at the link, the piston and cylinder were horizontal on this engine. The bike vibrated so badly it fractured the mounting holes on the license plate as I was riding one day. I was lucky that the plate bounced on the pavement a couple times and made a clattering noise so I could go retrieve it. I mounted it upside down with the holes on the bottom of the plate and it fractured and left the bike again, this time making no noise at all. I had to go to the DMV and get a replacement plate, which I mounted on a rubber blanket to resolve the problem.

In High School I rode and loved the Harley Davidson Sportster. At the time, it was sort of the "Sport Bike" in Harley's corral... smaller than their big bikes, but powerful enough to out-accelerate almost everything else on the road.
New Sportsters look like those old bikes but are much improved... they vibrate less, don't leak oil, and you no longer have to risk injury by kicking a lever to bring the engine to life. As you can see in the linked photo, the Sportster has a two-cylinder engine in the form of a "V", with a 45 degree angle between the two cylinders. Old Sportsters vibrated just about as badly as the single-cylinder Sprint. Today's Sportster engines have counterbalancers to negate some of the vibration.

Now I'm gonna rattle on about stuff I only partially understand...
Apparently "felt" vibration in engines is the result of the weight of lots of moving parts rapidly changing direction in the engine. The main vibration you feel is called a "primary vibration". There is also a "secondary vibration" that you can feel. There are tertiary vibrations and so on, but those dwindle to the point of not being discernible. Engine manufacturers worry mainly about primary and secondary vibrations.

Pizza Bike has a two-cylinder V-Twin engine. The V between the cylinders is measured at 90 degrees, so some actually refer to this engine, (and a similar configuration on Ducatis and Aprilias) as "L" twins. These bikes have virtually no primary vibration. There is some secondary vibration that can be felt, but it is MUCH less than the vibration felt on say, a Harley.

The other bike I was considering was the BMW horizontally-opposed twin. Called the "Boxer" engine, vibration is mostly canceled out on this engine because the two pistons move in equal and opposite directions. Here too, there are secondary and other vibrations, but those are minor in nature.

For me it boiled down to this:
I've never owned a BMW or Moto Guzzi. Both bikes have "shaft" final drive systems.
I like the sound of the V-twin over the opposed twin.
My choice was made. Time will tell if my decision was a good one.

But my studies also turned up a fact that I need someone to help me understand.
The following illustrations are horsepower/torque charts, the first is for an Aprilia motorcycle, the second is for a new Corvette. Take a quick look at both:

Now look again at where the torque curve meets the horsepower curve on both charts-
5250 rpm!
Apparently all engines that turn at more than 5250 rpm will have curves that mate up there.
I need someone to explain the "What" and "Why" of that phenomenon, please!


Timothy Frazier said...

Interesting. I thought surely you must have stumbled on a coincidence, but I went and checked the dyno chart for the Triumph Rocket III at http://image.motorcyclecruiser.com/f/10733113+w750+st0/r3-dyno-xl.jpeg.jpg and guess what?

I too would like to know why this is. Obviously there's some law of pysics at play, similar to the maximum rate at which a boat hull can be pushed through the water, no matter how much more horsepower you add to it.

Timothy Frazier said...

Here's an apparent explanation...but I am severely challenged in the math department and still need an interpreter:


Bob Barbanes said...

Man, I only WISH my Sportster had counterbalancers in the motor! I think only the Twin Cam 88 has 'em. All they did with the Sportster from 2004-on was rubber mount the engine. My '05 is smooth at certain rpm ranges, but at others it still vibrates like a jackhammer, just like my old solid-mount. I worry about the various frame welds and such, but that's my helicopter pilot paranoia creeping in.

Good luck with the pizza bike! I love it! I hope you spend a lot of great, fun miles on it. We're lucky here in the south that our riding season can last all year :)

Greybeard said...

Rode a buddy's 1200 Sporty last year Bob and it sure seemed smoother than the bikes I rode years ago. (I guess that rubber mounting DID do some good.) When I commented on it, he was the one that thought they had added counterbalancers.
I'd still buy a Sportster in a second if I could find one priced right.

Retirement in Destin is in the not-too-distant future. Mom is still in P-Cola...
Watch for that transverse-V when you're out and about.

Greybeard said...

Okay, I'm now in the old position of "I see it. I know it's important. But I need someone to explain why."
At the article here they state the way to find horsepower is to use this equation:

HP = Torque x RPM ÷ 5252

But the question remains and I'm sure it's as confusing to most, as it is to me:
Why are horsepower and torque equal at 5252 rpm?
Dig people... dig!

Greybeard said...

Really intersting stuff on the subject here.

Anonymous said...

This is properly termed an "artifact." The problem is that apples and oranges are plotted together. One HP is defined as 550 Foot*Pounds per Second. That works out to HP=Torque*RPM*5252.118 after getting the units correct. The curves will always cross at the same point.