Volvo's chief engineer (engines) has just retired. It seems a good time to take a look at Volvo's engine-design philosophy.
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VOLVO has long been associated with diesel engines in general and turbocharged diesel engines in particular. One man was largely responsible for the engine development programme which has brought so much success to Volvo, not only in Europe but also world-wide, and that man is Bertil Haggh.
Bertil has recently retired from his post as chief-engineer — engines at Gothenburg, and at a recent press gathering in London to mark the occasion, he outlined some of Volvo's design philosophy with special emphasis on overall engine efficiency.
A look first at some Volvo history would be useful. The Gothenburg company started to produce diesel engines immediately after the last war, with the biggest one being an indirect injection design producing 100 kW (135bhp) at 2200 rpm from a capacity of 8.7 litres (530 cu in). This was known as the VDB and was followed in 1950 by a direct injection version designated the VDF. At the same time the capacity was increased to 9.6 litres (586 cu in) — the same as the present 100 series engines.
The VDF had an alloy crankcase with forged main bearing caps but problems with ovality caused Volvo to develop a new version with an integral cast iron block. This used wet liners and was designated the D96AS. It was with this engine that Volvo began its investigations into the science of exhaust gas turbocharging.
The problem with being pioneers in any field is that there is usually a shortage of available hardware and this was true in the case of Volvo. The only turbocharger at that time which was anywhere near the right size for a heavy lorry was one de veloped by Eberspacher in Germany.
The first turbocharged engine (the TD96AS — the 'T' stood for turbo) ran in 1954 and produced 138 kW (185 bhp). By this time Volvo was convinced about the long term future of turbocharging, and engine development from then on reflected this. The TD96B developed 145 kW (195 bhp) and this was followed in 1963 by the 'C' version which gave 172 kW (230 bhp), Volvo initially ran into problems with maximum cylinder pressures at this rating, and so the compression ratio was dropped from 17 to 15 to 1.
Up to this point, all turbocharger development had been concentrated on getting more power from the existing size of engine. However, there is more to the turbocharger than that. Apart from the aforesaid increase in performance, the increase in air flow improved the combustion which reduced the exhaust emissions, both visible and invisible. On top of this, the noise level could be reduced by reducing the ignition delay.
A completely new series of engines was started in 1965 and this included the TD100A engine which retained many of the dimensions (including bore and crankshaft) of the earlier engines. Even at this time, the turbocharging technique was still used by most companies primarily to increase the power output at the top end.
In the early '70s however, expressions such as "high torque rise" and "constant horsepower engines" began to creep into the development engineer's vocabulary. In theory, such an engine could reduce the number of gear changes necessary, which implied a simpler transmission and a cost and weight reduction. But to utilise the intermediate engine speeds, where the specific consumption is better, was not as simple as it sounds. Because of the axle ratio requirement for a reasonable top speed, there was not enough power in the mid-range.
This was improved partly by the efforts of the turbocharger manufacturers and partly by those of the engine builders. Some manufacturers produced high torque rise engines by simply restricting the peak power output (Figure 1). Although this did not satisfy the demand for improvement in the mid-range, it did at least improve the overall fuel consumption because of the limited top speed and lower engine friction.
Bertil Haggh summarised the development of the diesel engine with the help of a diagram (Figure 2). This shows, albeit in an exaggerated manner for the sake of clarity, how an engine with the same rated horsepower can have vastly different characteristics thanks to turbocharging techniques.
Curve (1) indicates a naturally ispirated engine with practically s linear power curve. Curve (2) is ypical of an early turbocharged sngine with the emphasis on in;reased power at the top end. :urve (3) shows how progressive engine and turbocharger levelopment can improve a given engine throughout the speed range while still retaining he same peak power.
The torque curves provide an Dven more dramatic illustration pf the difference in performance Ind this leads on to a topic Nhich has often been mooted by vehicle designers in the past, lamely, is it sensible to base egislation on rated output (ie bhp/ton)?
In Bertil Haggh's opinion, the question is whether it will be necessary for manufacturers to specify the tractive effort of a vehicle or to specify both the rated output and the torque rise. Peak power is not really the deciding factor on the ability of a heavy vehicle to maintain a reasonable across-the-ground performance — and that is what a power-to-weight ratio purports to do.
At the end of last year Volvo put itself in one of those difficult situations for marketing departments: its latest engine, the new TD100GA, developed less power than its predecessor the TD100B. As Bertil Haggh put it "can you call this development?" But read on ...
Figure 3 shows the tractive effort required to overcome rolling and air resistance at various inclines for a 38 tonne articulated outfit. The actual tractive effort curves for two different F1Os (one with the TD100G and the other with the 100B) have been superimposed.
As can be seen, both engine options can provide a road speed of 105 km/h (65 mph). The extra tractive effort of the 100B above 1900 rpm does not give a higher top speed with the specification selected for good economy. However, at lower speeds, the 100G is very much superior in tractive efforts.
A comparison of the curves show that the 150 rpm reduction in maximum rated speed for the 100G is more than compensated for by the hefty increase in mid range tractive effort. And the average heavy lorry spends a lot of its time in this middle range.
The practical result of all this theory is that the usable speed range — or "drivability" — is increased, which in turn means fewer gear changes and the ability to hang on one gear higher than before.
Providing an interesting insight into current Volvo thinking on engine development, Bertil Haggh said that if more top end power was required for a specific operation, then a larger engine should be used. At the higher engine speeds necessary for the 100 series engine, it would not be operating in the most economical range.
Volvo is one of the few menu facturers to release details of how its fuel consumption figures vary with engine speed and load. These are usually expressed in the form of a mussel diagram — often known to the UK engineers as iso-loops or even "onion curves".
The aim of every engine designer is to increase the size of the area 'of maximum efficiency — shaded in Figure 4. As a rule of thumb, a specific fuel consumption of 210g/kWh is equivalent in Olde Englishe to about 0.345 lb/bhph.
The art of specifying a vehicle to achieve the lowest fuel consumption is directly connected with the ability to provide the correct gearing. "Provide" is the important word. It is not only necessary to calculate the correct gearing — it must also be available in the metal.
Not surprisingly therefore, Bertil Haggh is a strong supporter of Volvo's policy of designing and producing as many components in-house as possible — known in modern jargon as "vertical integration".
At a time when more and more manufacturers are moving away from this• policy (Leyland being a classic example), it is interesting the Volvo is still firmly committed to in-house manufacture.