Step by step explanation of primary and secondary engine balance:    • ENGINE BALANCE: Inline 3 vs. Inline 4...  

Today we're hitting on all sixes as we explore the engine balance as well as the strengths and weaknesses of the four most popular six cylinder engine configurations, the inline six, the v6, the vr6 and the flat six.

Let's start with the inline six cylinder engine. We already covered it twice in our videos and many of you by now know that the inline six is a beautifully simple and beautifully balanced engine configuration.
It's beautifully simple because it needs only 1 cylinder head and only 1 or two cams. It also needs only 1 exhaust manifold. It's only real downside is that it's long so fitting it transversely is extremely difficult and it needs a relatively long engine bay to fit in longitudinally. But other than the length it's very hard to fault the inline six.

When it comes engine balance the inline six is essentially two inline three engine's mirroring each other. You may remember that the inline three cylinder engine has a primary imbalance in relation to it's center of gravity due to it's odd number of pistons. This imbalance is especially apparent every time the first or last cylinder fires. When cylinder one fires the force pushing the piston down in this direction creates a reaction at the other end of the engine and tries to yank the engine upward, in the opposite direction. The third piston can't cancel this force out because when 1 is at TDC, 3 isn't at bottom dead center, it isn't doing the „opposite thing“ in order to be able to cancel out what cylinder 1 is doing – the final result is that the inline three rocks in relation to it's center of gravity.
But the inline six doesn't. It doesn't because the inline six is two inline three engine's mirroring each other so the primary imbalances of each individual inline three cancel each other out.
When it comes to secondary imbalance the inline three doesn't have problems there because different pistons are at different parts of their stroke which means the there are no significant secondary imbalances in the inline three. The inline six of course inherits this characteristic as it consists of two inline three engines.

Now the V6 engine. Last time we learned that separating an inline engine into two banks of cylinders meant that we had to select an appropriate angle between the two banks. The correct bank angle for a V engine that wants to use shared crank pins always equals the firing interval. Because we have six cylinders that's 120 degrees. Unfortunately a 120 degree V6 is impractical for packaging. It's almost as wide a flat six while also being a lot taller. This is why we have to settle for a narrower bank angle which is usually 90 or 60 degrees for most V6 engines. But when we do this we can't have both shared crank pins and an even firing interval. To have an even firing interval a 90 or 60 degree V6 must employ split crank pins. Opposing piston rods are offset by what's called a splay angle. The splay angle makes up for whatever is missing from the bank angle and ensures an even firing interval just like in an inline six.

Now the VR6. The best way to explain the VR6 is to imagine it as the child of an inline six father and a V6 mother. A child whose goal was to inherit the good and drop the bad genes of each parent.
VW developed the VR6 with the goal of making it compact, like a v6, but without the double cylinder heads, cams, exhaust manifolds and other components all while preserving the inherent balanced nature of the inline six. So how did they do it? Well they did it by creating what's a essentially V6 but with an extremely narrow angle between the banks. Instead of 60 or 90 degrees, a vr6 ENGINE HAS only 10.6 or 15 degrees between the banks, bringing them so close to each other that you can cover all the cylinders with a single, slightly wider, cylinder head. Yes you need slanted pistons to make it happen, but it works.

Our final configuration is the flat 6, or more accurately a boxer six. Not every flat engine is a boxer engine but all relevant modern mass produced flat six engines, like those made by Porsche or Subaru, are boxer sixes. In order to be a boxer, a flat engine must have the pistons moving in and out in unison. In order for the boxer thing to happen each piston has its own crank pin and the crankshaft looks like this. A flat engine can't be a boxer engine if the pistons share a crank pin. An example of a flat engine that isn't a boxer is the Flat 12 in the Ferrari Testarossa.
Anyhow, you're often going to hear how the boxer six engine is perfectly balanced, and it is, although there is a bit of a catch.

A special thank you to my patrons:
Daniel
Peter Della Flora
Daniel Morgan
William
Richard Caldwell
Pepe
Brian Durning
Andrew Ruud
Brian Alvarez
Holset90

D4A merch: https://teespring.com/en-GB/d4a-merch
Patreon:   / d4a  

#d4a #enginebalance #enginebootcamp