MadSci Network: Engineering

Re: What are the benefits of a V shape combustion engine from a transversal one

Date: Tue Dec 19 14:35:33 2000
Posted By: Mike Scannell, Powertrain Development Engineer, Ford Motor Company
Area of science: Engineering
ID: 974060647.Eg


Thanks for writing. It sounds like there are two questions you may have been asking, so I'll try to answer them both for you. There are two sets of configurations of an engine in a vehicle. The first is how the engine sits in the vehicle, and the second is how the engine itself is configured.

As far as the engine's in-vehicle position, there are basically two directions that the engine can sit: transverse or longitudinal. In other words, the engine can be orientied left to right, or front and back. This is usually related to where the engine is relative to the wheels that are driven. In front wheel drive cars, the engine tends to be transverse mounted. The engine will be mounted left to right (or right to left depending on the vehicle), and then the transmission will be mounted right to left--kind of looping back. This keeps everything up front in the vehicle to allow the front wheels to be driven. Rear wheel drive cars on the other hand tend to mount their engines longitudinally. The engine is oriented toward the rear of the car, followed by the transmission, and then a drive shaft behind that. Of course, there are other places to mount the engine besides the front. There are also mid and rear engine vehicles which add more possible combinations of engine placement, orientation, and drive wheels. But it basically all boils down to where do you want to put the weight (an engine and tranmission are quite heavy), how much room do you have, and what wheels do you want to drive.

The other type of engine configuration is how the cylinders are actually laid out in the engine block. Here are the most common configurations:

This is the most simple configuration with all the cylinders in a straight line, and all pointing in the same direction (usually up and down).

Half of the cylinders point in one direction, and the other half point in another direction forming a V-shape. The angle between the opposing banks of cylinders is usually 60 or 90 degrees for a V6 and 90 degrees for a V8.

Flat or Horizonatally Opposed
This could also be thought of as a V engine with an angle between the banks of 180 degrees. These engines are normally set horizontally, with half the cylinders pointing to the left, and the other half pointing to the right.

There are other types of engine configurations as well including radial (common in aircraft), rotary (like the old Mazda RX-7), and W (I believe I saw one of these in a Volkswagon).

The major performance trade off between the different types of engines is vibration. Depending on the number of cylinders and the cylinder configuration, an engine will tend to vibrate in different manners. In other words, what we're trying to do with different engine configurations is to make the forces produced by a bunch of pistons moving up and down cancel each other out, so the engine does not have any net vibration. For example, a V6 engine will not be very well balanced with either a 60 or 90 degree bank angle (but it's worse at 90 degrees). But an inline 6 cylinder will be fairly well balanced. Sometimes, manufacturers add balance shafts to engines to help improve the balance in an inherently unbalanced engine.

The other consideration that is usually taken into account when choosing an engine configuration is packaging. In other words, how well will the engine fit into a vehicle. A V6 is a fairly small engine, but an inline 6 cylinder engine is starting to get quite long. A straight 8 is so long, it's rarely even used in automobiles anymore. A flat engine will be much wider than an inline or V engine, but it will also be much shorter. These kinds of things must also be taken into consideration when choosing an engine for a vehicle.

Well, I hope that answers your question. If you want to learn more, you can try this link: "How A Car Engine Works". They have more information about the basics of how an engine works, and they also have some animated pictures of straight, V, and flat engines.

Your Mad Scientist,
Mike Scannell

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