Longer than you think?

Wednesday, January 08, 2014

Tags :  engine-structure

For race vehicles, in whichever motorsport they are used, the typical attributes are that they must be powerful, light and driveable. According to the engine used, ‘light’ can be linked to the overall mass of the engine, which depends largely on configuration and engine length – shorter usually equals lighter, of course, and by how much depends on the type of engine.

Therefore, during the concept design phase of the engine a lot of energy and focus is directed at keeping it as short as possible. A number of considerations on this issue have been discussed in earlier articles under this RET-Monitor keyword, but this article will look more closely at what actually determines the length with rising maximum combustion pressures.

In general, two design areas determine length – the upper side of the engine housing the cylinders, and the lower side of the engine housing the crankshaft. Both have their own issues in relation to the engine performance, and with increasing performance (and peak combustion pressures) the balance between them has shifted.

Looking closer at the upper side of the engine, the length is determined mainly by the bore and the distance between the cylinders, including the wall thickness and the coolant jacket between adjacent cylinders. Depending on engine type, the ratio between bore and stroke determines the bore.

For naturally aspirated engines this is typically as large as possible, since large intake and exhaust valves can be used in order to get as much air as possible into the cylinders and therefore maximum power output. Given the peak combustion pressures of around 90 bar max, the load from the piston and con rod on the main bearings is rather limited, which means bearing width is not an issue in relation to cylinder distance, taking the crank web thickness into account as well.

Looking at engines with forced induction, this changes things a little. From a certain bore-to-stroke ratio, a given bore size is derived, and since we are forcing air into the cylinder, the valve diameter is less of an issue. However, because the peak compression values are far higher (at around 130-140 bar) for a spark-ignited turbo engine, the resulting load through the piston and con rod will require much wider bearing shells. These, combined with the higher strength crank webs, will lead to wider main bearing distances.

Next we have the compression ignition engines, where peak combustion pressures have been exceeding 200 bar for some time now. Here the combustion loads on the piston are at such a level that the con rod and crankshaft bearing shells need greater material properties and dimensions, in order to withstand the loads on them by the combustion. Apart from material upgrades and detailed shape modifications, the width of these bearings has indeed grown and, in combination with the crank web widths, require more engine length than the minimum required piston distances at the upper side of the engine do. So, with these engines the crankshaft length is actually dominating overall engine length these days.

Exactly where the balance lies needs to be determined for each individual engine, but when moving up in terms of combustion pressures – as in turbodiesel engines – it needs to be taken into account.

The cases described here show that with the combustion pressures these days, especially with the entrance of highly loaded diesel engines into the racing scene, there will be no let-up in striving for higher efficiencies, and will continue for as long as internal combustion engines are raced. It is only when the new ‘green’ fuel initiatives fully take over or when we will run out of oil that this will probably no longer be a problem, but until that day... .

Written by Dieter van der Put

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