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Crank mechanism 4
| Piston used in Formula 1 . . . |
The engine displacement is, of course, the sum of the individual displacements. We intend to continue listing the bore and stroke on our website for as long as possible. Why? To trace the origins of the
engines. Here’s an example: The Jeep Renegade comes with a four-cylinder engine and a displacement of 1,332 cm3, and this time it’s even available as a PlugIn Hybrid
Electric Vehicle. The Mercedes A 250e is also a PHEV.
What stands out is that both are listed as having exactly the same displacement. Should they have the same engine? But the Mercedes engine is actually made by Renault, and Chrysler is on its way to forming a
partnership with PSA. So that doesn't really make sense. So I quickly checked the bore and stroke: 72.2 mm × 81.4 mm for the Mercedes and 70.0 mm × 86.5 mm for the Jeep. So it's not the same
original engine.
And how did this strange coincidence come about? We don't know why both 32 cm3 remain above 1300, either. This is problematic if a displacement tax is still in effect. Possibly for sporting events too, though
those likely aren't planned. But the approximately 330 cm3 single-cylinder displacement, which is considered something of an ideal size, is, surprisingly, viewed much more favorably than the
widely used 500 cm3.
This may give you an idea of the leeway you have when designing a completely new engine. However, it was often used in the past to offer different power ratings with as little effort as possible. In many cases, increasing
the displacement was more cost-effective than switching from a single carburetor to a dual or register carburetor. However, the stroke was usually kept the same, because changing it would have ended up costing too
much.
What was required was a base engine with a small displacement but a big gauge for boreholes. This refers to the distance between the centerlines of adjacent cylinders. If you subtract the bore from that, what remains is
the pure material between the cylinders. It shouldn't be much less than 5 mm, though of course that depends a little on the workload. You also need to pay attention to the cooling channels, which, at least for now, no longer
fit directly between the cylinders.
So, in an engine family designed from the ground up, you choose the gauge for boreholes so that you can still enlarge the bore. There have been countless examples throughout history where certain increases in
displacement simply weren't factored in, for example, Ferrari with its first twelve-cylinder engine. This is also because people want an engine like this to be as short and as light as possible, especially
the less powerful ones.
So once the limit has been reached in the gauge for boreholes, the only option left at first is to increase the stroke. So we need a new crankshaft with a greater center distance between the main and connecting rod
bearings (see image below). It becomes both cost-effective and unusual when the connecting rod bearing is turned out of center to such an extent that a larger center distance results. As you might have guessed,
increasing the displacement while weakening the connecting rod bearings isn't exactly ideal.
So a new crankshaft is needed, and sufficient space must be created for it at the same time. That would be almost a complete redesign. Now you may understand why, in the past, some designs, such as those with four
cylinders in a row, never achieved the two liters that might have been theoretically possible. In that case, the other configuration with carburetors, or a higher RPM range, is more cost-effective.
Because this aim is sometimes difficult to achieve, the saying, 'Nothing can replace engine displacement', has endured for so long. In principle, until the boost kicked in, though with the drawback that
the compressor consumed more fuel and the turbocharger caused a delay in acceleration from the bottom up. If electrical engineering hadn't provided a solution here, this demand would still be valid today.
We can't possibly finish talking about 'bore and stroke' without looking at the extremes. First of all, we have truck engines to offer, which are easy to work with because, these days, they’re almost exclusively
six-cylinder engines with displacements of up to, say, 16 liters. Now, to make the calculations easier, we could simply assume a stroke and bore of 150 mm, and we’d get the right answer right
away: 15,896 cm3.
That would be nice, but here, too, it’s important to note that a truck engine operates extremely fuel-efficiently at very low RPMs for its size, yet it must not show any weakness when accelerating
its heavy load. So it's more about stroke than bore. You could try doing a little puzzle.
In Formula 1, it's the other way around. In the past, this machine has reached extreme rotational speeds of nearly 20,000 rpm. To keep the piston speed at a manageable level, it was common practice here to
use a bore at least twice as large as the stroke. So if you use a bore of just under 100 mm and a stroke of less than 50 mm, you'll end up with an eight-cylinder engine with a displacement of about 3 liters, a figure that has
long been the standard.
And as you can see, when we plug these values into the formula for average piston speed, we end up with a value that’s roughly double what was long considered the gold standard for production engines. It's unbelievable
how the mechanism held up. No effort was spared, and they even used extra-powerful engines for the qualification. No wonder the costs eventually became too much for them.
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