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Library Home Page > 1st Generation (1983 to 1993) > Engine
Inline-4 Cylinder vs V-4 Cylinder Engines
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Inline-4 Cylinder vs V-4 Cylinder Engines

Jeff Pierce wrote:

> > I just got my bike back from the dealer. I thought it ran somewhat rough at low idle. He said it ran as well as any Venture he'd ever ridden. Is a V4 inherently rougher than an inline 4? If a dealer couldn't find anything to charge me for does that mean the bike is in great shape?

> > Keith

> My '83 and then '87 Ventures were always smooth at idle, or runnning down the road. Had Several other inline 4's, Honda and Kaw, the Venture is much smoother.

No, an inline is inherently smoother than a V-4. On an inline, a spark plug fires every 180 degrees, but a V-4 is not that smooth, especially the 70 deg V in the venture.
Consider a single cylinder 4-cycle engine, like the briggs and Stratton on your lawnmower. Lets say that 0 degrees will be top-dead-center (TDC) at the beginning of the intake stroke. Now, rotate the crankshaft through 180 degrees and the piston will be at bottom-dead-center (BDC) and at the beginning on the compression stroke. At 360 degrees the piston is again at TDC, the spark plug fires (well really 20-40 deg earlier, but let's not get lost in details) and you get power. At 540 degrees the piston is back at BDC and ready to start the exhaust stroke. At 720 degrees, the piston is again at TDC are ready for intake, just like 0. So, to work, you need two complete revolutions of the crank (720 degrees) to complete one power cycle. So the mixture fires once every 720, with each pulse being 720 degrees apart. If you think about this, the engine only "creates" power during the power stroke. That means that for 720 degrees of crankshaft revolution, you only are getting power for 180 degrees of that, or 25%. In order to keep the engine running for the remaining 540 degrees, the engine must have a large flywheel to store energy in. So, when the the piston fires, about 25% (really a little less) of the net energy produced is channeled out the crankshaft to the accessory, a mower blade, a pump, a generator, whatever. The remaining 75% is channeled into the fly wheel. For the next 180 degrees, 25% of the energy in the flywheel is used up to keep the crankshaft spinning and to deliver power to the accessory. After 540 degrees, all energy has been taken out of the flywheel, but it is time for the power stroke once again. For 180 degrees the crankshaft is accelerating (as energy is being added to the flywheel) and for 540 degrees, the crankshaft is decelerating (as the flywheel is supplying the energy.) If everything is in check (that is the engine is producing exactly enough power to the accessory, no more, no less) then the instantaneous rotational speed at any given point will be the same from cycle to cycle. If the engine is making more power than the accessory needs, then more energy will be stored in the flywheel than is taken out, and the average speed of the motor will increase. If more energy is being pulled from the flywheel than is being generated, then the average speed of the motor will decrease. If much more energy is pulled from the flywheel, then the motor will stop. (i.e., you hit a stump with your mower)
Now, consider an inline 4. The crankshaft will be ground that when the crank is at 0 degrees, you will have two pistons at TDC and two at BDC. One TDC piston is ready for intake, while the other is ready for power. This means that for any given slice in time, there are always all four cycles taking place simultaneously.
Conversely, at even given slice of time, there is always exactly one cylinder in the power stroke. When you get to a 4-cylinder engine, then you can pretty much get rid of the flywheel. Generally, the gear that the starter acts on, as well as the mass of the clutch is all the flywheel you need. So, if you start at 0 degrees, #3 has just fired, and #1 is on the intake stroke. After 180 degrees, #4 has fired, and #1 is on the compression stroke. At 360 degrees, #1 fires, and at 540 degrees #2 fires, and #1 is on the exhaust stroke. This leads to a very smooth application of power. Assuming a steady average RPM, the instantaneous rpm at every 180 degree interval will be the same. Finally, you get one power pulse every 180 degrees. (BTW, the normal firing order is 1-3-2-4, not as I have illustrated.)
Now, continue this line of thinking, but now take two inline 4 engines, let them share a common crankshaft, and place that at 90 degrees to each other. Presto, you have a "normal" V-8, i.e. a 350 chevy. They work just the same as an inline 4, but now at 0 degrees, you have one cylinder starting the power stroke, and one half way through with it. So, the instantaneous rpm every 90 is exactly the same on an engine running at a constant average rpm. The V-8 is probably the most "perfect" engine possible as far as smoothness and constant power. The next better would be what is called a W-12, where you have 3 inline 4's all sharing a common crank, but oriented at 60 degrees from each other. But, you are getting diminishing returns and packing becomes a real problem. Even better still is rotary used on old time aircraft, but for a given displacement, they must be at over twice as tall and a V-8, and are just not practical for things that don't have a propeller. But I digress.
Now, lets look at the venture. It is a 70 degree V-4. What you have is two inline twins sharing a crank and then oriented 70 degrees from each other. (again, I will used a simplified firing order) At 0 degrees, #1 is ready for intake, #2 is 110 into it's power stroke, #3 is 110 degrees into it's compression stroke, #4 is ready for compression. 70 degrees later #3 fires. At 180 degrees #4 fires, even though #3 still has 70 degrees of it's compression stroke left. At 360 degrees, #1 fires. At 540 no plug is ready to fire and 70 degrees go by without a power pulse. Then at 610 degrees #2 fires.  So, on an inline four, the plugs fire 0-180-360-540 all spaced 180,180,180,180.  On the venture V-4, the plugs fire 70-180-360-610 spaced 110,180,250,180.
An exaggerated example of V technology is the Harley engine. There it is a 45 deg V, with the two cylinders firing 45 degrees apart, 0-45 and spaced 45,675. That is why they sound like they are hardly running. A Harley idling at 800 rpm will sound like a Yamaha twin idling at 400 rpm.   The Yamaha picks up a little of that rumble during that 250 degree span with out a power pulse.
So, in answer to the original question, an inline 4 is inherently smoother than a V-4. But, with all that being said, I have to agree with Jeff, that my Venture is the smoothest bike I have owned at cruising speeds. I think that is more in the design of the bike (rubber isolation, weight of the bike, etc.). When I first test rode my Venture at the dealer (the first time I had ridden a V) we had a terrible time getting it started. It seems like someone was sitting on a bunch of the bikes looking them over and left the key on, thereby draining the battery. (OK, I confess, it was me) So when I took it out and rolled on the throttle, I though it was missing (because of the rumble, I thought it had dropped a cylinder) due to the dead battery. But, it ran much smoother at speed than my previous two KZ's.
After we bought the bike and got the battery straightened out, (and the "problem" didn't go away) I realized it was due to the cylinder configuration. Now that I know it is not something wrong, I kind of like that rumble. It has that attitude of a mini-hog when pulling away from a light. :-)
Whoops, that got a little long. Occupation hazard of being a former automotive engineer I guess.

Larry Piekarski - '88 Royale


Last update: 11:58 AM Sunday, September 26, 2004

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