Foreign News
By Peter Chinn, MAN 1979

DAVIS' DOUGHTY DIESELS; in the October issue we promised to give our impressions of one of Bob Davis' new large size diesel conversions This followed our remarks about the lack of success that had attended early European attempts at large displacement engines using the model compression ignition or "diesel" process.

We have now had the opportunity of examining the Davis Diesel Development conversions of the Italian SuperTigre ST.35 and Japanese O.S. Max40-FSR and Mav60-FSR engines, and have run some brief tests on the DOD Max40-FSR. On the basis of the 40-FSR tests, it has to be said that this engine makes mincemeat of most previously held convictions in regard to large diesels and, at the same time, confirms the feasibility of converting modern ringed glow engines to diesel operation.

For example, the Davis 40-FSR is NOT spiteful to handle, intolerably harsh running or lacking in power, as one was accustomed to find with medium and large size diesels in bygone years. Moreover, it responds to the throttle in a manner that betters anything we have previously encountered with a diesel.

Our feeling is that the main advantage of a diesel conversion of this type is that it expands the function of the basic (glow) engine to cover hitherto unexploited prop sizes. Because, via the variable compression ratio, one now has control over ignition timing, it is possible to safely load the engine with larger diameter props. Bigger diameters can be used to provide the greater low speed thrust that is needed, for example, to unstick a heavy scale model. Of course, the engine does not develop as much power when it is loaded down to a lower speed, but what really counts in this situation is not how much power the engine is delivering, but how much static thrust the prop is producing, on that power, to get the model rolling and up to takeoff speed. Here, there is no substitute for a large diameter prop that will take as big a bite of the air as possible.

Of course, revs can be kept up, to some extent, by reducing prop pitch. But the point to remember is that, with most model engines, maximum torque is developed at well below the revolutions at which maximum brake horsepower is delivered. Sometimes at only half the peak bhp speed. Especially when a muffler is added, this may mean that loading the' engine down from, say, 12.000 to 8,000 on a suitable prop may reduce power output by 25 percent, but increase static thrust quite appreciably. In certain instances, it may even be practicable to go down to less than 7,000 rpm: but, when we are dealing with modern high speed glow engines, which deliver their peak bhp at speeds in excess of 15.000 rpm, we are then running the rink of pre-ignition and overheating by overloading the engine in this way.

Just how much a diesel conversion may help in such a situation may be judged from the fact that the OS. 40-FSR (with which most users probably use a 10x6 for acrobatic work or an 11 to 12 inch diameter for scale models) proved capable of turning 14 inch props without protest when converted to diesel. We first checked the DDD 40-FSR on a 12x6 Top Flite maple, which it turned at 9,300 rpm. This was only 200 rpm short of the figure obtained on the same prop when using glow ignition and 5 percent nitro fuel. Switching to a 14x6 Top Flite maple, which was the largest prop we risked using (for test purposes only I with glow ignition, revolutions were exactly the same (7,400 rpm) with both diesel and glow ignition.

All tests were carried out with the standard OS. Type 743 muffler fitted. Other props tried on the DDD converted 40-FSR included a 14x4 Top Flite maple which it turned at 8.500 rpm, and a 16x4 Top Flite maple which the diesel swung, with no sign of distress or overheating, at a steady 6,700 rpm.

Throttle response was excellent: the engine idled safety at under 2.000 rpm on the OS. Type 4B carb, yet recovery to full speed was always reliable. Even after a lengthy period of idling. This contrasts with the usual tendencies with throttle equipped diesels, which are apt to misfire or cut out, when the throttle is reopened, due to the cooling off that has occurred during idling and the consequent retarding of the ignition timing. With the DDD 40-FSR, there was a short period of misfiring only if the engine had been idling for a long period and the throttle was opened abruptly, after which it soon picked up to full speed again. It the idling period was fairly short (say, half a minute) or if the throttle was reopened gradually, the recovery to full power was immediate.

Compression adjustment is effected by turning the large Allen cap screw, on top of the cylinder head, with the Allen key provided. It is a bit difficult, sometimes, to insert the Allen key into the hexagonal hole in the top of the screw when the engine is running and vibrating somewhat; for convenience, when making quick and precise compression adjustments while testing, we would have preferred a compression screw with its own tommy bar or lever. However, it has to be admitted that the screw looks neater and, once the setting needed for one particular prop size has been established, readjustments can be kept to a minimum. The usual procedure with diesels, when using big props (it is different on lighter loads), in to start on a high compression setting and to then back off the adjustment as the engine warms up, since the increasing cylinder temperature has the effect of overadvancing the ignition timing. However, with the DDD 40-FSR it was found that, if the throttle was closed to about one third open position and the engine spun over with an electric starter, it could be restarted on the running setting. It would first run in a series of bursts, gradually evening out into continuous running as it warmed up, and the throttle could then be gradually advanced to full power as the engine reached its normal running temperature.

We did not find it necessary to prime the cylinder at any time. A sufficiently rich mixture for starting could be introduced into the engine in one of two ways. Either the carb intake could be finger choked (with throttle wide open) and sufficient fuel sucked in by grasping the prop firmly and turning the shaft over two or three times, as recommended in the DDD instruction leaflet; or a finger could be momentarily placed over the intake (throttle at reduced starting setting) when the electric starter was Applied. This latter course of action is, perhaps, best left until the user is well versed in handling the engine, as it is quite likely to burst into life when the "choking finger" is withdrawn.

In the past, there has been a widely held view that all diesels need a very close fitting lapped ringless piston in order to obtain the level of piston seal necessary to raise the ether based fuel mixture to its self ignition temperature. The DDD 40-FSR totally disproves this theory: the excellent seal of the well' fitting OS. aluminum piston and its pegged compression ring is fully adequate. The engine gives a reassuring diesel "plop" as it is flipped over compression and Bob Davis' warning about carelessly flipping the prop when there is any risk of fuel remaining in the engine is well advised: despite thirty years' experience with diesels, we forgot, just once, and earned a well bruised knuckle as a reminder! We should, perhaps, mention that our dieselized 40-FSR had already had three or four hours' running as a glow engine, prior to conversion, so the ring was nicety bedded in,

These DDD conversions of larger engines do not feature the "loose" contra. piston and fluorocarbon disc seal of the original DDD Cox conversions described in the April 1977 Foreign Notes column. Nor do they have a cast iron contra piston tightly fitted into the upper part of the cylinder bore like most conventional model diesels. Instead, the conversion uses a cylinder head that is modeled on a glow type head. For example, the head for the 40-FSR has a sloped squish band surrounding a bowl shaped combustion chamber. The squish band is actually slightly wider than that of the glow head (4.2 mm instead of 3,7 mm) and the underside of the contra piston forms the roof of the 12.7 mm dia. combustion chamber. The contra piston, like the head, is machined from 2024T4 aluminum bar stock and is fitted into the head with two high temperature o-ring seals. 'The compression screw runs in a stain less steel helicoil thread insert. It is at arrangement that works extremely well, the contra piston moves smoothly without any tendency to stick when hot; and we would guess that the bowl and squish band configuration, with only the bow volume variable, is probably instrument tall in promoting smoother combustion and thereby avoiding the excessive harsh running tendencies that have been characteristic of large displacement diesels using conventional full bore contra pistons.