THE ubiquitous inline-four engine is the mainstay of many car manufacturers. But why did it become so commonplace and why not other configurations? Nowadays, some manufacturers are trying out other, more economically produced configurations. But the most common ones produced currently, from inline-fours to V-eights have their advantages and disadvantages. What is not so obvious is that you can make the common, or garden, inline-four produce as much power as a V-eight. It just takes ingenuity and, of course, money.
Four-cylinder inline four
The inline four-cylinder engine was, and still is, a staple of many car manufacturers. The main reason is largely because it’s so simple: one cylinder bank (usually cast iron), one cylinder head (aluminium) and one valve train.
It is usually small and compact, even those with large capacities, and easily fits in nearly any engine bay. It can be positioned in front (longitudinally or transversely), in the middle or even in the back. The light weight of the inline-four is due to having only one exhaust manifold and one intake manifold.
One cylinder head means there are fewer moving parts than engines with multiple cylinder banks. This also simplifies maintenance because the cylinder head is the highest point; spark plug replacement and valve train access is very easy. Vibrations are minimal because the outside two pistons move in the opposite direction of the inside two pistons.
All this makes the four-cylinder inline engines incur low manufacturing costs.
However, secondary vibrations are not balanced, which limits the size of the engine. Rarely will an inline-four exceed 2.5 litres with some having 3 litres. These big-capacity engines will often require balancing shafts to cancel the vibration caused by the secondary imbalance.
Another disadvantage is a higher centre of gravity compared with some layouts, reduced somewhat by angling the cylinder block. The long crankshaft means the engine is also not as rigid as other shorter crankshaft layouts.
Horizontally-opposed (boxer) configuration
Not a common layout, except when the air-cooled Beetle was a volume seller, but an excellent configuration from an engineering point of view. It is especially attractive due to the easy packaging it allows. Subaru places it under the front bonnet whereas the Beetle and Porsche has it in the rear.
Primary and secondary vibrations are well balanced, meaning the boxer is a smooth engine. This also allows for lighter components such as the pistons and the crankshaft, resulting in less power-sapping inertia. The boxer has an inherently low centre of gravity which allows for better handling.
However, boxers are very wide engines. A shorter stroke and clever cylinder head design reduces this disadvantage somewhat. Two cylinder heads (and sometimes two cylinder blocks) make the engine more complex and more expensive to manufacture. The offset required in the crankpin produces a slight vibration due to rocking couple.
Maintenance can be challenging, if packaging is tight. Removable panels in the engine bay are a regular feature. The Beetle just requires a few bolts removed before the engine drops out, thus this is not a disadvantage in its case. Conversely, doing a valve adjustment on an Impreza is a challenging prospect.
Inline straight-six
The straight-six is simply the result of adding two more cylinders to an inline-four engine. The favourite layout of engineers and many famous cars feature inline-sixes. From BMW, Jaguar to Nissan, the straight six has many advantages.
It is superbly inherently balanced. Each piston is only 60 degrees of crankshaft rotation from each other, making the inline-six almost vibration free. Just as the inline-four, the inline-six also makes design, manufacturing, construction and maintenance simple.
The simple design also makes V12s and Flat-12s a simple extension of the six, as they are simply two inline-sixes patched together.
Packaging can be difficult due to the length of the crankshaft. Front -wheel drive? Not an option. Just like an inline-four, the inline-six also suffers from a high centre of gravity.
V-Six and V-Eight
To allow a six-cylinder engine to power a front-wheel drive car, it needs to be configured into a vee layout. The V6 is a more common layout than an inline-six. This is due to the compact packaging and ease for both front wheel and rear wheel drive vehicles. The rigid engine block allows more displacement to be used, thus more power. So rigid is a V6 that Formula 1 teams use the engine as a stressed member of the chassis, even mounting suspension arms directly to the engine block.
The only disadvantage of V6 engines is the added complexity and cost of two cylinder heads. If the V6 is of a small capacity, added rotational inertia will sap a little bit of power. A slightly higher centre of gravity (compared to a boxer) is a small disadvantage as well. Anything other than a 90-degree vee angle means that the secondary imbalance requires additional weight on the crankshaft.
The logical extension to the V6 is an additional two cylinders to make a V8, which results in a much larger engine but a smoother one. The sound from a large-capacity V8 is also a contributing factor of its ongoing development, especially in the American market. Add another four cylinders and you get the darling of the Italian manufacturers, the V12. Minus two cylinders and well, you get the point.
There are, of course, the other, less common configurations, such as the rotary engine, inline-five and W-eight, but that’s another article in itself.