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/: ô wU °I (oJ —71
THE diesel-engine scene is not one which changes rapidly from year to year. "Evolution not revolution" sums up this section of industry, where a current production design can trace its ancestry back a long, long way. The Leyland 600/680 and its many derivatives (both inside and outside Lancashire) is a classic case.
Another example is the 14-litre Cummins engine which is currently popular in its Big Cam E290 form. This started life as a 12-litre (742cuin) unit in the early 1940s when it developed 150kW (200bhp) in naturally aspirated form. In 1982 the latest version of this basic engine — now in 14litre form following a bore increase in 1950 — develops no less than 355kW (475bhp).
This particular engine, designated the NTC 475, is notable for being the first production engine for automotive application to use two-stage turbocharging (that is, one turbocharger feeding into another).
The basic performance figures for the engine are impressive. It develops the 355kW (475bhp) at 2,100rpm with a peak torque of 1,940Nm (1430Ibft) at 1,400rpm.
In theory it is perfectly feasible to increase the power of an engine by increasing the turbo boost and the fuel delivery. Obviously there are limits as to how far this can be taken but the principle holds good.
A higher degree of boost can be achieved in two basic ways: by a larger single-stage turbocharger or two smaller ones in series. However, the single-turbo avenue does present certain difficulties as it has a relatively narrow flow range for the boost required and thus a narrower engine-speed range.
Using a two-stage system, on the other hand, increases the engine operating speed range thus giving greater overall flexibility. It also divides the work done by the compressor into two stages and allows this to be done using existing components. Thus, two current designs of turbocharger can be used rather than designing a new, larger, unit to do the same work.
On the thermal and mechanical side, the twin-turbo system lowers the top speed and the mechanical loadings for both the compressor and the turbline as well as decreasing the thermal loading on the highpressure compressor wheel. The Cummins two-stage system uses a Garrett AiResearch unit for the lowpressure stage followed by a Holset HC3 for the high pressure. The term "two stage" effectively describes how the system works in that the lowpressure turbocharger feeds into the high-pressure unit. The maximum practical pressureratio for a single turbocharger about three to one, but by running two in series an overa pressure-ratio of six to one cat be achieved.
With the NTC 475 engine, Cummins claims a final boost pressure of approximately 60ii of mercury. As with any compression process, a great amount of heat is generated sc an air-to-water charge cooler i incorporated to lower t'-=: temperature of the inleal dir an thus increase its mass.
The actual charge-cooling layout is similar to the E400, being incorporated in the inlet manifold, although more tube are included to give an extra 21 per cent cooling area. It is certainly an effective arrangement as Cummins claims an engine inlet temperature of around 93°C (200°F).
The same two-stage system also used on the exhaust side where the engine's exhaust gE drives through the highpressure turbine wheel before passing through the lowpressure turbo and out throug re vehicle's exhaust.
The significant feature about le choice of turbochargers is at both the Holset and Garrett ruts are straight off the shelf id are not special designs. There are some detail )ecification changes however; If example, the HC3 has a )mpressor cover in spheroidal raphite cast iron instead of the ormal aluminium. This is to icrease the safety factor in the dmittedly unlikely) case of a urst. The compressor wheel is lade from stainless steel in )ntrast to the alloy used for the .andard component. The irbine is the same as that used tr the E290, being made from iconel, which is a high nickel loy.
The latest development of ummins' long-serving 14-litre igine has a compression ratio f 13.7 to 1. As one would expect ith one basic engine and niersl levels of turbocharging, le compression ratio decreases the specific power output icreases. Thus the current E290 has a CR of 15.3, the 350 14.3, while the 370 and 400 both share the same compression ratio of 13.9 to 1.
Cummins claims that because of the slight decrease in compression ratio and retarded timing, the maximum cylinder pressures on the twin-turbo engine have been kept to the same level as the E350's.
Although the weight increase is inevitable in such a case, the difference is not in fact that much. The standard E400 weighs 1,194kg (2,610Ib) compared with the 1,245kg (2,7441b) of the 475, a reasonable trade-off for an extra 75 horsepower.
The valve specification for the 475 is identical to the E400 on the exhaust side (stellite) but the inlet valve composition is markedly different, with a massive increase in the chromium content. The E-series inlet valve is of chromium/silicon steel with proportions of eight and three per cent respectively. The 475 inlet spec, however, is 20 per cent chromium and two per cent silicon with the addition of one per cent nickel.
Testing an engine is a protracted business even for one whose basic design goes back a long way. It is impossible to hurry bench testing: 500 hours is 500 hours, arid with a 24-hour day it does not take a genius to work out the absolute minimum length of the bench test programme.
With the two-stage turbocharged engine Cummins ran three engines on the bed for 500 hours in a thermal overload
test, each with an ambient air intake temperature of 66°C (150°F) and a water outlet temperature of 98°C (210°F). By way of comparison, this is 50 per cent above the recommended air-inlet temperature and 4°C (7°F) above the maximum permitted water-outlet. temperature.
For mechanical overload, two engines were run, again for 500 hours each, at 10 per cent overspeed and 10 per cent overfuel. I could not prise any power figures out of Cummins for these engines!
The third laboratory test was to evaluate cycle endurance using three engines running for over 1,000 hours each. Each cycle consisted of seven minutes at rated horsepower, seven minutes at peak torque, three minutes at high idle and three minutes at low idle. During the test, each engine completed over 30,000 cycles.
The 475 twin turbo engine has been available for UK chassis manufacturers since June 1981. Several of them are looking at it for specialist operation and export. Because of its notinsignificant torque output (1,430Ibft or 1,940Nm) the 475 requires the beefiest Fuller or Spicer box to go behind it.
The two-stage turbocharging concept has also been applied to the Cummins vee engines, although the plumbing is, not surprisingly, more complicated. The intake airflow follows the same flow pattern as the inline engines but to simplify the exhaust manifolding as much as possible, the exhaust from the high-pressure turbocharger on one cylinder bank crosses over through the low-pressure turbo from the other bank.