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KTM RC 390 with exhaust gas charging pump in the driving report

ALP instead of turbo

In contrast to the turbocharger, the exhaust gas charge pump uses a membrane to build up positive pressure on the opposite inlet side with the aid of the exhaust gases. We drove a KTM RC 390 with such an innovative shock wave charger.

Arno Hofmann, a physicist, has a very special ALP dream. A bold but hopeful one. In a nutshell, this dream consists of marketing your development for the series production of a two-wheeler manufacturer who produces small to medium-sized single-cylinder engines in large numbers. In order to demonstrate the potential of his shock wave charger, he presented MOTORRAD with a converted one KTM RC 390 available for a short test.

KTM RC 390 with exhaust gas charging pump in the driving report

ALP instead of turbo

How it works is not easy to understand

As the lead photo shows, the exhaust gas charging pump sits on top of the tubular space frame, where the tank is mounted on the series RC. As a replacement for the standard tank, Arno Hofmann constructed a stainless steel container that fits between the gearbox and the shock absorber in the frame. Although the ALP and its components are all custom-made and therefore do not have the perfect processing quality of a mass-produced product, they are carefully crafted. The charger housing was made from polyamide in a 3-D printer, the complex exhaust system from cleanly welded stainless steel pipes.

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Improvised temperature control: The upper number indicates the exhaust gas temperature, the middle one that in the charge pump. Below is the difference between the two values.

It is not easy to understand the meaning and function of this pipe loop. The way in which the supercharging works should not be imagined in such a way that the hot exhaust gas flow presses directly on the membrane in the pump. This would burn immediately. Instead, the hot gases first flow into the thick pulse tube, where they can be cooled down by the airflow and slowed down to a standstill. Between the hot exhaust gases and the membrane of the charge pump, there are cooler exhaust gas molecules that act as heat buffers and at the same time pass on the kinetic energy of the exhaust gas flow to the membrane. Only about five percent of the heat from the freshly flowing exhaust gases reach the charger. Despite this, the polyamide housing, due to its low thermal conductivity, reaches critical temperatures of over 95 degrees when driving at full throttle for long periods. For larger series, a housing made of cast aluminum promises better heat dissipation.

Back to charging

In the cover on the fresh air side of the charge pump there is a system of diaphragm tongues that open in opposite directions. Some allow ambient air to flow into the fresh gas chamber, while others allow compressed air to flow into a pressure accumulator, which is located in front of the throttle valve in the direction of flow and causes the higher gas throughput. After the kinetic energy of the exhaust gas molecules has been used, the large membrane in the charger is pushed back towards the exhaust side by a spring. Fresh air flows into the charger opposite. At the same time, the exhaust gas molecules are set in motion against their original direction of flow, but now much more slowly. If the exhaust valves were still open, the exhaust gases would be pushed back into the cylinder head, but since these are closed, they flow through the so-called pulse switch into the actual exhaust smoking pipe and thus into the open. The test vehicle does not have exhaust gas purification using catalysts.

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When measuring power, the KTM with the exhaust gas charging pump pushes eleven hp and eight Nm more.

So far, so confusing. But what is the point of this system, which uses relatively simple components – pipes, membranes, chambers, hoses – to set complex gas dynamics in motion? As a glance at the performance diagram shows, it delivers eleven hp and 8.1 Nm more peak power and torque than a standard comparison motorcycle of the same specification with correspondingly voluminous curves. From around 4,200 rpm there are significant advantages for the ALP-KTM. Compared to a new KTM RC 390, which was revised for 2018, they are even bigger. In addition to the ALP, Arno Hofmann changed the timing of his engine by rotating the camshafts and lengthened the intake manifold between the throttle valve and intake valve in order to increase the torque in the lower range. The higher gas throughput also requires a new adjustment of the injection. It is therefore not possible to determine which part of the higher power is due to the charging and which part is due to the other modifications.

Engine quite convincing

When driving, it is immediately noticeable how powerfully and spontaneously the response behavior of the ALP engine gets to the point. Just as clearly, however, is the undulating course of the torque curve. With diligent fine-tuning of the tuning, a lot could certainly be improved here; an effort that usually only well-known manufacturers can afford for large-scale production. There is further need for optimization in terms of noise development, because the plastic housing of the exhaust gas charge pump generates a powerful roar in the rhythm of the pressure waves. The Giannelli silencer is not exactly one of the quietest ones.

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The gain in torque enables a secondary gear ratio that is three teeth go along.

Arno Hofmann wanted to convert the gain in power and torque into a reduction in consumption with the help of a longer secondary transmission. Compared to the standard RC with the same gear ratio, this does not work. Compared to a motorcycle that uses a conventional drive to generate similar performance from a larger displacement, the ALP engine looks better. A Honda NC 750, for example, consumed 3.4 liters per 100 kilometers on the MOTORRAD test lap. This value is not based on a direct comparison, so the advantage should not be rated too high. On the other hand, you have to take into account that the ALP engine was developed by a small business, while the NC engines were designed and optimized for low fuel consumption with the concentrated development power of the world’s largest motorcycle manufacturer. While the ALP-KTM RC 390 was only able to distinguish itself to a limited extent through lower fuel consumption, its powerful, lively engine was quite convincing. You could mistake it for a spirited RC 450 or RC 500. And that’s not a bad basis for further development steps.

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