Sir, I was extremely interested in Capt. Moon’s article on “Diesel Engines for Sports Cars,” as I have had considerable experience of compression ignition power

units in road vehicles.

I have always understood that, owing to the fine limits to which fuel pumps and injectors have to be made, the cost of the injection apparatus alone is almost as much as that of a comparable petrol engine.

The above refers to engines of at least 3to 5-litre capacity, and if a smaller engine was required, even finer limits might be necessary with a corresponding increase in cost. I believe that it is necessary to run a C.I.-engined car for at least 40,000 miles a year before the saving in fuel cost nullifies the increased outlay on the engine.

Some of the C.I. engines I have driven were reasonably smooth at normal speeds, but nearly all had a bad vibration period usually arranged to coincide with tickover. Their chief advantage was that they were infinitely more reliable than the petrol engines they replaced, and for heavy goods or passenger vehicles the fuel saving was considerable.

Perhaps the following example of the saving effected by changing over to C.I. engines may interest your readers.

A fleet of public service vehicles-26and 32-seaters—were originally fitted with normal petrol engines of a very reputable make. The engines were governed to a maximum of 2,100 r.p.m., at which speed they developed approximately 60 b.h.p. The back axle ratio was 6.25 to 1 and maximum road speed 38 m.p.h. Petrol consumption averaged 8-10 m.p.g., and

at times oil consumption was such that it was necessary to top up the sump during the day’s run of 250-300 miles. These engines were replaced by Gardner oil engines developing 68 b.h.p. at 1,750 r.p.m., and the axle ratio raised to 5.5 to 1, which gave approximately the same

road speed as before, 38 m.p.h. The improvement in performance was incredible—hills which formerly required 2nd gear were climbed on top, and owing to the increased torque at low revs., the pick-up was improved.

The petrol engines had required fairly frequent valve grinding and decarbonising, but the oil engines did 40,000 miles between top overhauls and 180,000 before re-sleeving became necessary. The inereased reliability and freedom from ignition troubles, etc., was very marked, and it was indeed rare for a vehicle to be held up on the road with any form of engine failure. As a matter of fact, I cannot recall a single instance of bearing failure in the fleet over a period of six or seven years. These engines, which were installed in 1933, are still going strong after 12 years of hard work, which speaks for itself.

From the operator’s point of view the fuel saving was amazing. With a greatly improved road performance the fuel consumption averaged 18 to 20 m.p.g. of fuel oil against the 8 to 10 m.p.g. with the petrol engines, an improvement of 100 per cent. Oil consumption was slightly less, the vehicles requiring no topping-up of sump while on the road. Starting from cold presented no difficulty despite the fact that no electric starters werq fitted. A decompressor lever lifted the inlet valves and enabled one to swing the engine by hand. When the engine was turning nicely, release of the decompressor resulted in a certain start. With a very cold, new engine, where sufficient cranking speed could not be obtained by hand, I found the following method very effective :— A piece of rope was looped round the starting handle, which was engaged at about the 8 o’clock position, and with one man on the handle and one or two on the rope, the engine was pulled over full

compression and would start immediately. No heater plugs or other starting devices were employed. I have heard of drivers stuffing a petrolsoaked rag in the air intake and setting

it alight to heat lip the induction system when an engine was very obstinate, but have never found it necessary myself. Another interesting point about the Gardner engine is that the accelerator pedal is merely a speed control and is not connected directly to the throttle. The engine is governed both for tick-over and maximum revs. The throttle spring pulls the throttle open, reversing the usual practice so that with the engine stopped the throttle is always wide open. On starting up, as soon as the revs, reach the pre-determined tick-over speed, the

governor automatically closes the throttle to maintain those revs. If a load is applied to the engine the governor opens the throttle to maintain the revs, without any movement of the accelerator pedal.

For example, assuming the vehicle is cruising along in top gear with the accelerator pedal depressed half-way, this will give a road speed of approximately 20 m.p.h., i.e., half maximum speed and revs. If a hill is climbed the governor will open the throttle to maintain 20 m.p.h., it being possible to have the throttle fully open with the accelerator only partly depressed.

On topping the hill, if a descent lies ahead, the governor will close the throttle as the speed reaches 20 m.p.h. until it is possible for the throttle to be fully closed, although the accelerator pedal has not been moved from the mid-position.

I have handled 3-, 4and 5-cylinder Gardner engines on the road. The 3cylinder was a lusty motor but very rough, every individual firing stroke being felt. The 4-cylinder was much smoother, and the 5-cylinder was a delightfully smooth and well-balanced engine, the cranks being set at 72° to each other, giving an evenly-spaced firing order.

One 39-seater motor-coach I have handled had a 5-cylinder Gardner engine with an overdrive gearbox giving an extra super-top gear of about 2.8 to 1. I have had a road speed of 62 m.p.h. with the engine turning over at about 1,600-1,700 r.p.m.

I imagine that an engine such as the smaller 4-cylinder L.K. Gardner in a suitable chassis would make an interesting sports car, but a back axle ratio of about 2 to 1 would be necessary, or else a stepup in the transmission. This engine is a very advanced job with electron crank ease and sump, aluminium block with wet liners and aluminium head with phosphor-bronze valve inserts and injector housings. Capt. Moon spoke of the Leyland

coaches with hydraulic torque convertor. These are a sound, reliable job, but have one drawback. When in ” convertor ” there is a free-wheel effect, i.e., if the throttle is closed the vehicle will coast exactly as in a free-wheel-equipped car. In top gear there is a dog-clutch

engaged direct drive. On icy roads or descending steep hills it is not possible to engage a lower gear to assist in controlling the vehicle, so top gear has to be used, a point which many drivers dislike. Wishing all success to MOTOR SPORT. I am, Yours, etc.,

R.A.F. C. M. TAIT (F/Lt.).