................
For the model sleeve valve engine, the scaled down supercharger will not actually produce any boost; unless you run it at jet engine speeds. The model engine effectively becomes a normally aspirated engine, and the valve events need to change to reflect this. The 'hot' port (valve) timing of a racing engine would be undesirable. Likewise the port (valve) timing associated with a supercharged engine, operating at altitude would not be ideal either. For a well mannered and reliable normally aspirated model engine, running on the bench, a 'milder' port timing; with shorter opening times and lower overlap are called for............................
Mike
The above quote spells out my views on the valve event timing that will provide for a smooth running and well mannered model sleeve valve engine. Not the wild 'spicy' timing of a racing or supercharged engine, but a less aggressive, 'milder' set of valve timing events.
So, let me convert those descriptive names into numbers and charts. From these, we can directly compare some actual data and make a considered judgement; which can then lead onto the design work and drawings.
I have done an extensive trawl of the net and have found manufactures data on a number of engines. The valve event timing has been converted into a common format to ease comparisons. The valve event diagrams conveniently display the four Otto Cycle strokes (suck, squeeze, bang, blow) as one 360* rotation of the Sleeve crank, The crankshaft would have completed two complete revolutions during this time.
Note: most published valve (port) timing information give the opening and closing angles with respect to crankshaft rotation.
The diagrams I have presented show the port timing wrt. the Sleeve Crank or Camshaft angle. That's crankshaft angle divided by two.
Some Sleeve Valve engine port timing diagramsThe first diagram is for the Napier Sabre, supercharged, sleeve valve, in-line engine. This was one of the last sleeve valve engines designed, a very powerful engine. You will see, the valve event display all the characteristics associated with a highly supercharged, high performance engine. The inlet and exhaust ports (valves) are open for a very long period and there is considerable port (valve) overlap for scavenging blow through. The result would be a high revving engine with a narrow power band, Ideal suited to a powerful aircraft engine, which generally operate at a constant cruising speed except during the take off and climb.
The next diagram is for the Bristol Hercules, supercharged, sleeve valve, radial engine. This engine was produced in great numbers with a reputation for both power and reliability. It also has all the characteristics associated with a highly supercharged high performance engine. However the valve events are only slightly less aggressive when compared to the Napier Sabre.
Here is the port timing diagram for a normally aspirated sleeve valve engine. It is the work of Joachim Steinke from Germany. The port timing is noticeably less aggressive than the two supercharged engines above. The port opening times are much shorter as is the reduced port overlap duration. This engine is reported to start easily and run well.
This diagram is for the Barr and Stroud, normally aspirated, single cylinder motor cycle engine. The port events are significantly different to the two high performance, supercharged, engines above. In this engine, both the exhaust and inlet ports (valves) remain open for a much shorter duration and the port (valve) overlap is virtually non-existent. This 'milder' state of tuning would have given the Barr and Stroud engine the highly desirable characteristics of a wider power-band and a well mannered engine response from tick over to full power.
This diagram shows the port timing I am proposing to use on my single cylinder test rig. You can see the port timing is noticeably less aggressive than the two supercharged engines above. The port opening times are much shorter and the port overlap duration (20 degrees) is also reduced but not to the extent of the Barr and Stroud.
Some Poppet valve timings for comparison For comparison, here is the valve events diagram for the highly supercharged (2.3 bar) 1939 Mercedes Benz W165 Grand Prix racing engine. The inlet and exhaust ports (valves) are open for a very long period and there is considerable port (valve) overlap for scavenging blow through. However , the designers favored a longer inlet period to that of the exhaust. The result was a high revving engine with a narrow power band, Ideal suited to a powerful racing machine, which had the benefit of a 5 speed gearbox.
This diagram is presented by way of a bench mark. This diagram is for the valve events used in many of Edgar Westbury's, normally aspirated, poppet valve, engines. His engines were noted for their good behavior and ease of starting and have been copied into the design of many other miniature engines. The valve events may also considered to be 'mild', compared to the supercharged engines. The valve overlap period is slightly wider than the Barr and Stoud but still less than half that of the supercharged engines.
Here is the valve timing diagram for a Briggs and Stratton lawn mower engine. Similar valve events to those of the Barr and Stroud engine of the 1920's; but still being sold 100 years later, little has changed. It has similar short duration valve openings and next to no overlap
This is the valve timing used by the normally aspirated, quarter scale, Hodgson radial engines. This timing diagram was evolved following extensive testing of several different camshaft designs, which in the end resulted in a smooth running well mannered engine. The valve event timing is another 'mild' camshaft timing when compared to the 'spicy' timings used by most supercharged engines (whether sleeve valve or poppet valve). The valve open durations are relatively short and the overlap duration is very short, less than a quarter of the supercharged engines. Note the asymmetric timing of the EO and IC events, which favor a slightly longer Exhaust duration.
Although described as an industrial engine, I have used the Honda GX35 in all of my 1/6 scale model tanks (petrol powered Panzers). The piston diameter is 35mm which coincides with the model Hercules' piston diameter. The idle speed of the GX35 is a fast 3,000 RPM above that, it performs smoothly and powerfully. The high idle speed may be due to the unusually late inlet valve timing on what is intended as an industrial engine. However, Honda usually know what they are doing.
I will leave you to decide the characteristics you would prefer to have for your model engine. I believe that somewhere between the Barr and Stroud and Westbury would be most desirable and will progress my designs accordingly. If all fails, the sleeve ports can always be reworked to open up the ports towards the larger size and shape of the supercharged engine.
So there, at last, is some real data for you all to digest and think about
Mike
