THE D.D.E. – THE FORGOTTEN IDEA.

In my previous article on the history of the 6.354 engine I referred briefly to something called the DDE. This is worthy of an article to itself, since the story is technically very interesting although the project never reached production.

We need to start with the consideration of a basic compromise which affects every conventional internal combustion engine, whether diesel or spark ignition. These engines all have a basic drawback, in that their characteristics are not wholly suitable for traction applications. Whilst the steam engine and the DC electric motor have ideal characteristics for traction, with maximum muscle at low speed, this is not the case with the internal combustion engine where its ability to give good ‘step-off’ from rest is poor. Conventional engineering resolves this by the use of a gearbox and clutch. Many attempts over the years have attempted to address this, many using some means to boost engine performance such as supercharging or turbocharging.

In 1957, as the development of the 6.354 was progressing in Peterborough, a paper published by a Dr Glamann working for the Berliet company in France came to Perkins attention. A weekend visit by Gordon Dawson, the Perkins Technical Director, to discuss his work resulted in an invitation to become a consultant to Perkins.

Glamann’s design introduced an epicyclic gear train between the engine and a torque converter drive to the output shaft, plus a mechanical drive to a supercharger. The planet carrier of the epicyclic drive was connected to the engine output from the crankshaft, with the sun wheel connected to the supercharger via a gear train. The annulus of the epicyclic train provided the power output, coupled to the input end of the torque converter and thence to the drive to the road wheels. The differential effect provided by the epicyclic gear train divided the torque output between the drive to the road wheels and the supercharger.

This gave several valuable effects by (a) sensing the vehicle requirement for additional torque and thus driving the supercharger more quickly, increasing the ‘boost’ to the engine, and (b) when power requirement was lower by decreasing the supercharger speed and reducing the engine speed so giving increased economy and reduced engine wear. The use of the torque converter dispensed with the need for a clutch and also allowed multiplication of the torque output still further. In essence this power unit gave maximum supercharger boost when road speed was zero, simulating the steam engine’s high torque output for standing starts.

The Perkins application of this principle, known as the Differential Diesel Engine, or DDE for short, used the new 6.354. A Godfrey twin-screw compressor was mounted on the left side of the cylinder block, with a K.S.B. torque converter behind the differential. An air-to-air heat exchanger was added to provide charge cooling. Since the engine was going to operate at considerably increased power output (up to 170 BHP) a special fuel pump was designed by CAV to provide the increased fuel input needed, especially at low engine speeds.

Initial trials in a vehicle showed that the unit had promise, delivering improved over-the-road performance although not reaching full expectations. Refinements of the design were gradually introduced, with this effort running in parallel with the mainstream development of the 6.354 at Queen Street. It was found that there were additional features needed for effective use on the road. The first was the addition of a two speed and reverse gearbox: this allowed road performance to match that of a vehicle equipped with a ‘crawler’ gear. The second was the incorporation of a hydraulic retarder in unit with the gearbox: this countered the loss of engine braking resulting from the differential drive, and in fact gave enhanced braking performance so that the standard service brakes were used for low speed and emergencies only.

Road tests confirmed that the DDE could handle loads up to 40% greater than a conventional engine of equal top-end power, with especial benefits over hilly routes whilst achieving better fuel economy at high speed and load.

The inherent characteristics of the drive also gave benefits in terms of lower component temperatures, reduced noise and the advantage of two-pedal control. (One of the strange aspects of the vehicles was the fact that to a bystander the engine note dropped as the vehicle accelerated away, due to the whine from the supercharger dropping as its speed reduced!)

The development of the DDE continued through the turmoil of the Massey Ferguson take-over until about 1966. During that time the vehicle testing demonstrated the benefits quite convincingly, although there were a series of problems requiring modifications of the basic engine components to withstand the much higher mechanical loads under extreme conditions. The writer recalls failures of cylinder head gaskets, pistons, bearings, valves and crankshafts (most of which might now be resolved by the application of current materials and designs!). The development of the high delivery fuel pump also required special attention from CAV to ensure reliable and matched operation.

The project was eventually dropped in 1967, due partly to pressure of other engine developments but also consideration of economics and other factors. Over its prototype lifetime the DDE had grown in complexity through the addition of the gearbox, retarder and special internal engine parts. This meant also that the size of the assembly had grown, especially in length where the full DDE assembly was more than twice the length of the base engine. The cost of the engine was also a factor, since compared to the cost of a larger conventional engine with a multi-range gearbox the DDE had become more expensive. It was true also that other engines were now available, and more effective multi-range or automatic gearboxes coupled to them. The potential for better fuel economy and reduced driver fatigue might have been used to counter these factors but management support was no longer there. With the change of directors in Engineering and elsewhere the death knell for the project sounded.

There had been plans to apply the DDE principle to the new V8.510 engine nearing production in 1967: indeed the crankshaft of the V8 was designed to cater for the increased mechanical loads involved! But it never happened and all physical traces of the DDE are now gone, although the drawings may still exist. Perhaps this was an opportunity missed, but who knows if a 21^st century version might prove both feasible and economical giving ease of driving on our busier roads in return for a little bit of sophistication?

© David Boulton July 2002