00:00 Intro
00:35 Motorcycle and car applications and firing intervals
05:00 Acceleration, Velocity and Force
07:58 Primary balance and vibrations explained
12:22 180° and 360° twins primary balance and rocking couple
16:10 270° primary balance
16:49 Secondary balance and vibrations explained
20:07 180° and 360° twins secondary balance
21:34 270° secondary balance and rocking couple
22:40 Balance summary and reasons why the 270° twin is so popular nowadays
25:25 Pumping losses


What is up engine heads, today we’ll be taking a very detailed look at the engine balance of parallel twin engines and we will compare the three different widely used parallel twin configurations that can be seen on motorcycles and even in some cars, the 360, 180 and 270 degree inline twin or parallel twin engines. Although this will be a very detailed and long explanation I promise that it will be organic and easy to understand for everyone and I guarantee that if you pay attention, by the end of this video you will have the satisfaction of a newfound appreciation for the reciprocating piston engine.

So let’s start from the basics. An inline twin or parallel twin cylinder engine is any engine where the there are two cylinders right next to each other in the same line using the same cylinder head. An inline twin can also be called a straight twin or a parallel twin engine because the two cylinders are in parallel.
The only other two cylinder configurations are the Vtwin and the flat twin. Unlike the inline twin both the v and the flat need two cylinder heads as the two cylinders are physically separated from each other.

Now when it comes to the inline twin cylinder engine there are three widely used configurations. 360 degrees (most British bikes from the 1930s and onward such as the Norton Commando, BSA A65, Matchless, Triumph Bonneville, BMW F800GS, Kawasaki W800 and the Fiat Twin air engine ),

180 degrees(Most two cylinder Japanese bikes from the 60s including the Honda CB450, Suzuki GS400, Yamaha XS500, with modern examples being the Ninja 650 and the Yamaha R3 )

and finally 270 degrees (Honda Africa twin, Honda NC750, New Honda Rebel, Aprilia RS660, Tuono and Tuareg, BMW F900XR, All Triumph twins after 2016 like the street twin, street scrambler, bonneville, bonneville bobber, yamaha mt07, yamaha r7, yamaha tenere, Royal Enfield continental GT and interceptor ). The degrees refer to the firing interval of the engine in crankshaft degrees.

First up we have the 360 degree twin. In the case of this configuration the pistons move up and down together. So a cylinder fires, the engine rotates 360 degrees and then the other cylinder fires, again we rotate 360 degrees fire the first cylinder and so on. In other words the engine has an even fire interval and fires once every revolution.

Next up we have the 180 degree twin. In this case we have 180 degrees of separation between the two crank pins. This of course means that when one piston is at TDC or top dead center the other piston will be at BDC or bottom dead center. And as the engine runs it looks like this. A cylinder fires, the engine rotates 180 degrees and then the other cylinder fires. Obviously because we’re talking about four stroke engines here when the second cylinder fires the first one is still the other is on the exhaust stroke which means that the first cylinder must complete exhaust and also do intake and compression before it can fire again. Each stroke is 180 degrees which means that the engine must rotate another 540 degrees before it can fire again. The result is that the firing interval is uneven and goes 180 540 180 540.

Finally we have the 270 degree parallel twin. In this case there is 270 degrees or 90 degrees separation between the two crank pins, depending on how you look at it. The result is that one piston always trails the other by half a stroke or by 90 degrees or crankshaft rotation. When it comes to the firing interval of the 270 degree twin we have the following scenario: A cylinder fires, the engine rotates 270 degrees and then the other cylinder fires. When the second cylinder fires the first one will have completes half of its exhaust stroke which means that it has to complete the remaining half of the exhaust stroke and then do intake and compression which is 90 + 180 + 180 which equals 450 degrees. Meaning that the engine has to rotate another 450 degrees again before it can fire the first cylinder again. The end result is again an uneven firing interval of 270 450 270 450 270 450. This is the same as a vtwin which results in the 270 degree parallel twin having a similar soundtrack and character to a v twin.



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