Model Engine Development

Part 8

by Gordon Cornell

Automatic Induction

The Two-Stroke engine as we know it was patented by Joseph Day in 1892—the first engines were built in 1889. A US patent for the Day-Cock three port engine was granted in 1895. A further Day patent dated 1884 was for an improvement in flap valves applied to compressors (see Joseph Day 1855-1946 by Hugh Torrens—published by The Bath Industrial Heritage Centre, Julian Road, Bath BA1 2RH, UK). This is the first reference I have found to a flap/petal valve. The earliest personal experience of this induction method occurred in 1948 when testing a stationery engine with "hit and miss ignition" at Wimbledon Technical College. The fact is that automatic valves have been in use for over 100 years.

American manufacturers were early adopters of this type of induction system although similar types were produced by the UK and European manufacturers. In the USA, the reed valve form of automatic induction was mainly applied to 1/2A engines (0.049 cubic inch displacement) using glowplug ignition and added nitro methane. In the UK, nitro methane was prohibitively expensive, hence the diesel/compression ignition engine was the preferred option. The development of the Cox 049 TD front rotary engine changed the direction of induction development for high performance free flight engines after 1958. Effectively these are sprint type engines that are not satisfactory in control-line applications.

What has to be appreciated is that development is an ongoing, worldwide process. Rear induction engines were no longer fashionable for some applications while engine peak RPM was increasing. This lead to the premature conclusion that a reed valve would not be able to respond to higher RPM. This may have been a false conclusion. In my view, reed valve induction needs to be investigated in much greater depth as two stroke motorcycle engines now almost universally use this system. A Reed Valve provides a very flexible performance at modest RPM; an essential feature for beginners. Reed valve engines can be made to run in either direction which in some circumstances can be an advantage. Unfortunately under light load it can start and run in the reverse direction due to inadequate propeller inertia, so take care when starting.

Originally due to limited time and prejudice, I failed to fully investigate the application of Automatic Induction. The Frog 1.49 Vibramatic and ED Fury had obvious defects limiting performance. It was only later that I established the real problem with the ED Fury was the size of the cylinder ports. The design of the ED Reed valve variants was also severely compromised by basing these on existing Disc valve back-plates which did not place the reeds in the correct location in relation to the ideal gas flow.

The best commercial model diesel engine with a Reed Valve up to 1958 was the Taifun Hurricane; the Elfin 1.49 BB reed valve being bulky and heavy. The Oliver Cub Mk1 was front rotary, expensive, and not that special. My early experience with the Hurrikan lead to my not pursuing this line of development; Dave Platt and myself having produced 1/2A Team Racers using this engine which were flown at Beaulieu airfield (Hampshire) and Wanstead flats (London). David's model is in the Vintage list.

Auto Induction Theory

The theory is that a valve opens and closes in response to crankcase pump pressure. The maximum differential pressure during induction is unlikely to exceed 2 psi. Due to inlet flow, the valve closes when atmospheric pressure is achieved. Crankcase pump compression then takes place until the transfer ports open—the maximum compression pressure is unlikely to exceed 25 psia. Hence the sealing pressure across the valve is less than 14.7 psi (one atmosphere).

It has been implied that the inertia of the valve is critical if the engine is to run at high RPM. Hence reed valves have been made as light as possible in order to achieve this objective. Various materials have been tried—Beryllium Copper, Steel and plastics. Modern motorcycle size engines now apply reinforced plastics.

In practice, the petal valve will seal much better than may be expected. The incoming mixture is extremely wet thus creating a capillary effect between this and the back plate. A consequence of this is that there is a delay in response to adjustment of mixture strength, so when setting to a lean mixture, the engine may stop abruptly. This can make optimization difficult.

Performance Prediction

I have applied ICE to examine the potential of Reed Valve Induction. Performance testing of my reed valve replicas was much better than expected. It appears that the ability of the valve to respond to crankcase pump pressure may be beneficial. There have been some minor changes to the program hence slightly higher outputs may be predicted (some of the basic settings may be optimistic), however this is not a problem when applied as a comparator.

The first graph is for the Fury with an original cylinder and applied Induction Port Coefficient of 0.7. The second is for a Replica Super Fury fitted with a Reed Valve as per the included drawings. In this case an Induction Port Coefficient of 0.8 has been applied. This change has been applied to cater for the improved location of the valve—it is central directly under the piston. The prediction is not too different from a Cox TD 09 power curve so it may be feasible.

Observations

It appears that the ability of a reed valve to respond to crankcase pump pressure may have definite benefits. But are these commercially viable? The valve must be durable and not fail prematurely. Testing alternative designs and materials is required because the predictions imply an increased level of torque over the entire RPM range. Part of this is due to improved Mechanical Efficiency—there are no disc or drum friction losses. Because ME is improved, Brake Specific Consumption will be better. Extended testing of unobstructed choke systems such as the peripheral jet is required, the implication being that higher gas flows should be feasible.

Making a Reed Valve System

The Fury used the back-plate casting from the ED Bee. This restricted the position and size of the choke which attached using a 0.25 inch diameter ME—40 TPI thread, locked in position by a thin locknut. Unfortunately this is close to a back-plate fixing screw. For the Replica and development purposes, the inlet has been relocated to a central poison. The 0.25 inch diameter thread severely restricts the maximum choke size. With a conventional spray bar the fuel will enter the boundary layer of the inlet tract, thus restricting flow.

The threaded choke permits a peripheral jet system to be installed. With this arrangement, the inlet tract is not obstructed by the jet, hence a larger effective choke diameter may be employed and tested. This is a more expensive option which would have to be justified in that any improvement in power output could not be achieved by other means. Drawings for both systems are provided.

An alternative would be a fixed choke back-plate—this would be the least expensive option however the jet could only be fitted horizontally if gas flow was not to be compromised. Fitting the jet vertically makes access difficult—in some circumstances an extended needle would be provided. That's fine as long as the user has not cut this off for an alternative installation. You have to think the whole design through when designing down to a price. The reality is that rear induction engines will cost more than there counterparts.

Do we need the Reed Valve Clamp? This can be used to restrict the maximum bending stress that the reed/petal is subjected to. The lower the stress the greater the fatigue life, examination of an original ED Fury Clamp does not indicate this has been fully investigated. The clamp slot as manufactured does not conform to an arc profile which would reduce stress (see Machinery's Handbook reference Bending Moments of Cantilever Beams). Hence it only serves to reduce crankcase volume and direct gas flow.

Manufacture of the Back-plate, Chokes, Locknut and Jet Block is fairly straight forward. A shop-made soft collet can be made to hold the Reed Clamp for drilling and milling. This is shown in the photo sequence above.

In view of the fine thread for the Choke you will probably need to purchase a tap and die (the cost is minimal).

Design and Manufacture of the Reed

The original Fury reed was made from spring steel shim. Alignment of the valve is achieved by a semicircular arc opposite to the port petal. This aids assembly of the back-plate, reed and clamp. If you modify the shape make sure it can be correctly located during assembly. Having now made a limited number of reeds my impression is this is not the most suitable design to manufacture in the home workshop. A simple strip design would be easier to make. However it is important to establish how successful the original concept was.

Preparation

Consider how you intend to make the Reeds, several may be required—the selected material will change the process and tooling. The basic problem is the limited thickness of the material

Suppliers

Taps and Dies:
	Tracy Tools Ltd
	Unit 1, Parkfield Industrial Estate,
	Barton Hill Way
	Torquay, Devon	TQ2 8JG, UK
	Phone—01803 328603

Induction Review

We have now tried three Induction systems. With the aid of the ICE program we have been able to observe the relationship between cylinder porting and induction. Some engines have Piston ported Induction systems. Unfortunately with this arrangement the Scavenge/Transfer ports may be restricted by the Induction port required in the cylinder. Piston ported Induction systems are normally associated with Loop or Cross Flow cylinder designs (see diagrams in Part 5). These can perform very well when fitted with a tuned exhaust pipe. Unfortunately this can limit the useful RPM range. The Fury/Super Fury cylinder is not suitable for this line of development.