Analysis of a 4-stroke racing engine design project for karts – TKART

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

Talking about kart engines is always fascinating. The subject is not simple and opens up an infinite range of possible details, but, perhaps for this reason too, any new analysis can allow us to learn something new even for those who think they know everything about engines. Almost always, the articles concerning engines start from the analysis of a product or the description of an aspect of their operation.
This time, to try to give a different perspective, let's start from the design of an engine that has never been manufactured. This is the 4-stroke model with a cascade distribution of gears and finger rocker arms. Characteristics outside the regulations, but which give a particularly interesting analysis insights and food for thought.

Even the power is not regular, given that the configuration the analysis concerns is deliberately very radical, with the aim of exceeding the specific power of 200 hp/litre. Clearly, in this case it is not possible, or desirable, to present a detailed project of a finished product: it would be absurd to even think it, given the complexity of the subject! However, despite the limited nature of the approach, analysing the design of a potential engine, rather than a finished product, allows you to begin to understand how many and which aspects a designer/constructor must take into account in his/her work, such as problems to be faced and what the possible choices are that, inevitably, direct the entire project towards certain characteristics rather than others.

Form the start, the project provides a certain versatility about some of the chsoen solutions, an essential requirement to be able to make corrections in the development phase

Some general views of the engine positioned on the chassis of a kart

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

CYLINDER HEAD UNIT

Observing the drawings of the project, you will notice small signs of "milling" in the cylinders: they serve to avoid the interference of the valves, equipped with a wide diameter. Among the cylinders (drawn only schematically) there is a connection window that dampens the pressures and depressions due to the pumping of the pistons, which would create useless braking friction

ROCKER ARM

The finger rocker arm has an "S" design, necessary to prevent geometric interference with the spark plug wrench

SPRINGS

If the conical springs, initially included in the project, do not give the desired results, a more radical solution with double springs, one inside and one outside, that touch each other, is also included. In fact, it has been observed that the internal spring, coming into slight contact with the coils of the external one, helps to dampen the resonances. On the other hand, there is clearly more distribution frictions and, as a consequence, greater difficulty in achieving the rpm when the engine is located underneath

BEARINGS

The camshafts are only supported at the two ends by ball bearings: this is also possible thanks to the limited width of the motor. However, there are bearings made in two halves in the market which, placed at the center of the camshaft, would prevent the deformation of the same: a solution to be taken into consideration only in case this eventuality occurs

GEARS

The "cascade" configuration allows the maintenance of the correct meshing of the primitive diameters of the gears, even if changes are necessary, for example, the height of the cylinder head

The design of this 4-stroke engine shows a geometry of the ducts and an inclination of the valves with "formula one" angles. The camshafts rotate on ball bearings; finger rocker arms on roller cages. The spark plug has a M10 thread: a bigger model would have occupied uselessly useful space for the valves. After numerous studies, the choice of the rocker arm control was as a risers/timings compromise. Furthermore, this solution aims to reduce the alternating masses which cause high values of inertial forces as much as possible. As mentioned, the project aims to achieve maximum engine power and, therefore, the rpm will be high: for this reason the moving parts have been reduced as much as possible: the valve stem has a diameter of 4 mm. Unfortunately, due to the incredible inclination of the ducts, the valves are very long, but well guided. Also, the choice of using conical springs, with a smaller upwards diameter, allows having a smaller "spring guide" and, therefore, lower overall inertia, to the advantage of reaching high rpm. The possibility of using titanium (prohibited by the regulations) would have allowed a further increase in performance. The result of extensive studies is also the shape of the combustion chamber, which allows the achioevement of a high compression ratio (about 14:1 in theory).

DESIGNING A 4-STROKE RACING ENGINE FOR KARTS