Ask the Mechanic

Steve Thompson, Contributing Columnist

READER: I grew up with old, inline, 6-cylinder engines in pickup trucks and old grain trucks. I was always amazed at how smooth those old engines would run. No shaking, no vibration, just smooth. I see that now, some of the smaller trucks and cars are using the 6-cylinder inline engine. I test-drove a half-ton truck the other day with this engine, and even though it was fuel-injected and turbocharged with super horsepower and torque, it still had that unbelievable, 6-cylinder, inline smooth run about it that nothing can match. What makes that engine so smooth compared to other engines?

Steve: The 6-cylinder inline engine without a doubt wins the smooth blue ribbon. Two major things make an engine run smoothly: One is engine balance, and the other is power overlaps. These allow combustion forces and the pistons to naturally (without counterweight) mitigate vibrations. The inline-6 is balanced perfectly. The way pistons move, they balance each other with their up-and-down movement and sideways swaying. The even firing order of a 6-cylinder engine is a piston firing every 120 degrees, which is a perfect balance that allows one cylinder on a power stroke, when the next firing cylinder begins the next power stroke. It takes 720 degrees (two complete rounds of the flywheel) to complete a cycle. Divide 720 by 6, and you get 120 degrees, an exactly balanced firing sequence for smooth operation. Look at the legs of a three-legged barstool, and you will see how balanced they are around the circle. Yes, the 6-cylinder inline usually will have a harmonic balancer, but it is primarily to minimize shock force on the crankshaft. Other engines can also run smoothly, but they will require some sort of balancing.

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-- Email Steve Thompson care of askthemechaniccolumn@gmail.com, and be sure to include your contact information and phone number.

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