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Diesel practice during the ’thirties

These pages cover diesel practice in the UK during the ’thirties, and do not necessarily reflect current practice.

Diesel engine features

Differences between diesel and petrol engines

Vehicle diesel engines were constructed along the same lines as their much more numerous petrol counterparts. Like petrol engines, diesels could be built as two-strokes or four-strokes, they could be naturally-aspirated or supercharged, water-cooled or air-cooled. The main characteristic of the diesel was its use of the temperature rise generated by air compression to ignite the fuel in the cylinders, rather than the petrol engine’s electric spark. This difference in ignition method for diesels and petrol engines gave rise to several key differences between these types of engine, given below

  1. Diesel engines had much higher CRs than petrol engines, in order to raise the air temperature in the cylinders during the compression stroke to a level causing ignition. Typically, diesel CRs were about 16 to 1. Petrol engine CRs were about 5.5. to 1;
  2. To cope with the extra loading caused by high CRs, diesel engines were more strongly built than petrol engines, which made diesels heavier than petrol engines of the same power;
  3. Thanks to their ignition method, diesel engines needed no ignition system, doing away with HT spark ignition systems used in petrol engines (i.e. spark plugs and magnetos or HT coils and distributors and associated HT cables). The absence of a potentially troublesome HT ignition system was an advantage of the diesel engine;
  4. Thanks to their high CRs, diesel engines were more economical than petrol engines, and this was a major attraction for commercial operators. However diesel engines were more expensive;
  5. Instead of carburettors used in petrol engines to supply ready-mixed fuel and air to the cylinders, diesel engines used high pressure fuel injection into the combustion spaces to mix with air drawn through the inlet valves. Methods were devised to ensure good mixing of fuel and air in diesel engines to enable efficient combustion for best economy;
  6. The diesel engine’s combustion process was different to that of the petrol engine, as explained later.

Combustion processes

In a petrol engine, the combustible fuel-air mixture was ignited by the spark after the compression stroke, and combustion was rapidly completed before the piston had started to make any significant downward movement during the ensuing power stroke. Thus, combustion took place at constant volume, in which there was a rapid rise in pressure until combustion was complete. However, in slow-speed diesel engines, such as those used to power ships, fuel was injected into the high-temperature air in the combustion space after the compression stroke, whereupon, after a slight lag, spontaneous combustion took place. There was no pressure rise when combustion started and thereafter. Fuel injection (and hence combustion) continued while the piston descended, and the fuel injection rate was controlled so as to maintain constant cylinder pressure during combustion. Fuel was cut off while the piston was part way down the cylinder.

Constant pressure combustion, just described, was a feature of the Diesel cycle. However, small high-speed diesels used in road vehicles departed from the Diesel cycle and used a process of constant volume combustion (as in petrol engines) followed by constant pressure combustion (as in the Diesel cycle). This was one of the reasons why vehicle diesel engines were not regarded as true diesels and were therefore referred to as oil engines or CI engines. Incidentally, it may seem bizarre to refer to a multi-litre lorry diesel giving 150bhp at a leisurely 1,800rpm as small and high speed. However, compared to large marine diesels developing 10,000bhp at 100rpm, even the largest lorry engines were indeed small and high-revving.

Reportedly, at least one type of diesel engine ran with the complete combustion process taking place at constant volume, as with the petrol engine. This was the Packard DR-980 diesel aero engine in the USA in 1928, with a CR of about 18 to 1 and rated at 225hp@1,900rpm.

Fuel injection

Two methods of fuel injection for diesel engines were evolved, known as air injection and solid injection respectively:

  • air injection: fuel oil was blown into the combustion chamber using highly compressed air. Air injection was conceived as a means of injection by Dr Rudolph Diesel, and it was considered to be feature of a true diesel engine. For this reason, engines using solid injection (see below) were often referred to as CI or oil engines, as they were not seen as ‘proper’ diesels. Eventually, however, any CI piston engine was referred to as a diesel;
  • Solid injection: fuel was injected at high pressure into the combustion chamber using an injector pump. This was used on the vast majority of commercial vehicle applications and will only be considered in these pages.

A diesel fuel system, typically, comprised a fuel pump, an injector pump and one injector per cylinder. The fuel pump transferred fuel from the fuel tank to the injector pump, and the latter delivered fuel at high pressure to the injectors, from which fuel was injected into the combustion spaces. Injector pumps were engine driven and fuel pumps could be, although a vacuum fuel transfer system was sometimes used. On a six-cylinder engine, for example, the injector pump had six cam-operated plungers in a casing. Each plunger was allocated to supply fuel to a specific cylinder through a small bore pipe connected to the injector. A visual hallmark of a diesel engine was a rectangular injector pump fixed to the side of the engine, with a small bore pipe leading to each injector screwed into the cylinder head.

Each injector was actuated automatically by fuel pressure from the injector pump, and, typically, comprised a casing containing a small plunger forced upwards against a spring by oil pressure from its particular part of the injector pump. The injector’s plunger was attached to a needle valve which, when lifted, admitted fuel into the combustion space. The fuel from the injector was discharged as a fine spray, to promote good mixing with the compressed air in the cylinder so as to allow efficient combustion.

In the early says of diesel lorries, the injection timing (advance/retard) was controlled manually by the driver in much the same way as spark timing on a petrol engine. In due course, automatic means were devised for injection timing, as they were for petrol engine spark timing. Power on a diesel engine was controlled by altering the quantity of fuel injected during the power stroke. This could be done either by altering the injector pump piston stroke or by other means while retaining a constant injector pump stroke, e.g. by rotating the injector plungers using a rack connected to the accelerator, hence altering port opening times. Injector pumps were required to deliver precisely, consistently and rapidly tiny quantities of fuel at each injection, even after high mileages. They had, therefore, to be manufactured to fine limits in order to function accurately. It was a tribute to the engineering competence of those concerned that injector pumps giving consistently excellent results could be produced in quantity.

Fuel could be administered by direct or indirect injection, as given below:

Direct fuel injection: fuel was injected straight into the cylinder on direct injection diesels. Shaped piston crowns were sometimes used to produce air movement which, with a suitable injector nozzle, gave good mixing of the air and fuel, resulting in efficient combustion.

Indirect fuel injection: fuel was injected into a chamber located in the cylinder head; the chamber was connected to the cylinder proper by a passage. On a correctly designed system, this gave a level of air movement which allowed lower injection pressures to be used than for direct injection, with consequent beneficial effects on injector and injector pump construction. Alternative cylinder head chambers used for indirect injection are described below.

In comparison with one another, direct and indirect injection had their advantages and disadvantages. Both were used by the UK’s diesel engine industry.

Indirect fuel injection cylinder head chambers
Cylinder head chambers used with indirect injection evolved into three main types:

Pre-combustion chamber: the compressed volume of air after the compression stroke was shared between the pre-combustion chamber and the cylinder proper. On the firing stroke, fuel was injected into the pre-combustion chamber, where combustion started spontaneously and spread, via a narrow connecting passage, into the cylinder proper where most of the combustion took place;

Ante-chamber: this device had a larger volume than the pre-combustion chamber. The compressed volume of air after the compression stroke was all, as far as possible, contained in the ante-chamber. A well-known ante-chamber system was that due to Ricardo in the UK, in which a spherical chamber in the cylinder head was connected to the cylinder by a passage entering the ante-chamber tangentially, imparting rapid swirl to the air in the ante-chamber as a result of the compression stroke. This device was taken up by Thornycroft and other manufacturers;

Air cell: Air was forced during compression into a chamber (the air cell). This air was discharged into the burning mixture during the power stroke.

As with direct and indirect injection, each of the three cylinder head chamber arrangements mentioned above had their adherents in the UK. Thornycroft’s first engine offered to the market had direct injection and the next four (CIND4 and CIND6, followed by the DC4 and DC6) had indirect injection working in conjunction with ante-chambers (possibly of Thornycroft design). Developments of the DC4 and DC6 (DC4/1 and DC6/1) had indirect injection working in conjunction with Ricardo ante-chambers.

Environmental issues

Environmental concerns as we know them today did not exist during the ’thirties, and global warming due to man-made pollution was not an issue of public concern. Main concerns were vibration, smell and exhaust smoke. Vibration was unpleasant for drivers and was reckoned to be an issue even in 1938, by which time diesel vehicles were well-established. Smell and smoke were unpleasant for those who were near diesel vehicles. In November 1931, Mr Tom Thornycroft said that diesel engines were only suitable for long-distance work, because if there were too many diesel vehicles in crowded areas, the smell would put people against them. British manufacturer Tangye gave performance figures for its 9.2 litre six-cylinder diesel engine both for smoky and clean exhaust, depending on user requirements! Tangye’s engine gave 130bhp clean, and 137bhp with smoke. Manufacturers thought it worth mentioning if their engines had clean exhausts even at full load.


DC4 1engine - Developed from the DC4 diesel, the 5.26 litre DC4/1 four-cylinder diesel engine was introduced in late 1935 and gave 85bhp@2,200 rpm.

DC6 1 engine - Like its smaller DC4/1 brother, the 7.89 litre DC6/1 six-cylinder diesel incorporated the Ricardo arrangement of indirect injection.