Diesel Engines

Sir,

I am very much with you over the use of diesel engines in private cars, though I must enter one caveat against one line in your piece. This is where you say that the inventor did not help the search for improved fuel pumps and injectors by walking off the cross-channel steamer in 1912. His death made no difference, may indeed have helped, as the engine he invented relied on compressed air for injection and atomisation of the fuel, and being a very obstinate and pretty unpleasant man he set his face firmly against "solid" injection (by pump and injector nozzle) and carried on a campaign against it. This is why the true diesel engine could not be adapted to road vehicle work— ii is far too big and bulky with its compressors and air bottles, etc., and the system isn't suitable for working much above 400 r.p.m. I believe the air injection or true diesel system is still used for some big marine or stationary engines, but I am not sure. What we now call a diesel engine is really a development from the Ackroyd—patented, if my memory is right, in 1891, or a bit before the date of Rudolf Diesel's system.

I notice you say "to obviate the need for a distribution system the Hessleman 6-cylinder heavy-oil engine had a separate pump for each cylinder . . .". True, but so did all other solid injection diesels until very recently though, as the multiple pumps were usually in one housing, the fact is not immediately apparent. It was the Bosch or Bosch-type pump and injector system which made the diesel a practicable proposition for lorries, etc., in the late twenties, and if you dismantle a Bosch pump for a six-cylinder engine, say, you will find it has six barrels and plungers in one housing, worked from a little camshaft and all controlled by the same governor and rack-rod. It is only fairly recently that a practicable single-pump and multiple distribution device has been produced, and I must confess I don't know how they work. I do happen to know the Bosch-type well for one of my dreary factory jobs in the 'thirties, 1934 I think, was at the CAV Bosch factory in Acton where they had just started making diesel pumps and injectors, most of which went to AEC for buses and lorries. These were six-cylinder pumps but we also made single-, twoand four-cylinder pumps for other sorts of engines. I worked for a few weeks in the "lapping room", a glass enclosure in the main shop where the pump pluggers were lapped individually into their barrels. I had the job of testing the finished things on a simple gadget in which the plungers were worked up and down by a lever and each pump barrel had to reach a certain pressure on a gauge, 2,000 psi. I think, with only so many strokes of the lever. With a stop watch the loss of pressure was timed, and if the gauge dropped back so many pounds in so many seconds the unit was rejected. I forget the figures but it was a stringent test and the reject rate was pretty high—one of the reasons why the pumps were so expensive even in those days of cheap labour, as I think there were eight separate lapping operations; and where Rolls-Royce lap-in the control valves of their automatic gearbox with jewellers' rouge, this represented the antipenultimate lapping medium for the pumps. From the jewellers' rouge they moved on to tallow and from tallow to paraffin—and I don't suppose you can get a less abrasive polishing medium than paraffin. I expect it's all altered now and much more mechanised.

One of the great advantages of the diesel to meet the need for fuel economy is that we can have it now—or very nearly now. Small diesel engines have already reached the stage of being able to run up to 4,000 r.p.m. or more and with just a little more attention to the rather crude governing system commonly used (the lethal pneumatic governor is no more, I believe) most of the complaints of rough idling and diesel knock can be eliminated. With a compression ignition engine you not only get better thermal efficiency but can also have good lowspeed torque again—I know this is as near to your heart as mine. With what we know already I am sure it would be possible to produce a reasonably roomy four-seater (say of Daf size—Mini is too small for serious long-distance transport for four) weighing not more than 15 cwt. with a diesel engine of about 600 to 800 c.c. which could give 60 m.p.h. and 60 m.p.g., or 70-50 or similar combinations according to choice. As diesel combustion problems are in inverse proportion to bore diameters a two-cylinder would be preferable to a four-cylinder. Anyway a horizontally opposed twin is inherently better balanced than an in-line four, and the layout suits direct-air cooling which helps long-term reliability and saves a bit of weight.

Potbridge Anthony Bird