M.A.N. ENGINE EFFICIENCY IMPROVEMENTS
Page 78
If you've noticed an error in this article please click here to report it so we can fix it.
BY THE TECHNICAL EDITOR A S referred to briefly on page 9 of last week's issue, Maschinenfabrik AugsburgNtirnberg A.G., Germany, have been making some striking improvements to the performance of their M-series of automotive diesel engines as produced at their Nuremberg factory for use in vehicles built in the Munich plant. This engine design work has been carried out under the leadership of Director Dr.-Ing. Dr.-Ing. E. h. J. Siegfried Meurer, who developed the " M " combustion system about 110 years ago, this system having been used on all M.A.N. automotive engines since 1954, whilst patents on the system have been taken fon by several other engine manufacturers.
The " M " system has several advantages over more conventional diesel practices, including quiet running and the ability to burn a wide variety of liquid fuels. Stow fuel combustion is the secret behind Dr. Meurer's design, the fuel being injected onto the relatively cool wall of a spherical combustion chamber formed centrally in the piston crown, cooling of the crown being assisted by jets of oil directed from the crankcase at the rate of up to 0.5 gal. per min., whilst the injector nozzle is kept cool by using a copper insert at the nozzle tip to speed heat dissipation to the cylinder head.
Ideal Heat Flow
Indeed, the heat flow away from the pistons approaches the ideal pattern, and this cannot help but enhance the overall efficiency of the engine. By slowing up the evaporation rate of the fuel and allowing only a small quantity of the injected fuel to burn initially, combustion starts smoothly, and evaporation and combustion of the fuel which has been deposited on the wall of the chamber are accelerated by the rising temperature and the vigorous air swirl created by the specially shaped inlet ports.
This hot-air swirl characterizes the combustion cycle in the M-system_ Recently, however, M.A.N.'s engine designers have realized that more fuel could be burnt in the cylinder without the generation of exhaust smoke by making the motion of the hot-air swirl more violent, thus putting the air in the combustion chamber to better use so that the engine would give more power without requiring more air. This, in turn, meant that higher performance could be obtained without increasing the peak pressure in the cylinder.
a52 Development work showed that the kinetic energy of the air column in the helical intake ducts could be controlled by extending the pipes leading to the ducts, by careful timing of the valve opening periods and by use of paired inlet manifolds so that only three cylinders which followed consecutively upon each other in the inlet sequence were combined. Thus, the speed of the air through the intake duct was increased, and so it had a more energetic rotary motion after entry into the cylinder, this having the advantage that more air entered the cylinder each time also.
To avoid setting up unwanted natural movements of the air in the inlet manifolds, which would restrict these effects to a very small speed range, a compensating chamber is included in the inlet piping.
Engines incorporating this new system are known as the HM-series, and in all cases are distinguished from ordinary M-series units by the increased torque output in the medium:speed range and greater b.h.p. at governed speed. Internally, however, the reduced peak pressures relative to the power output should lead to proportionately increased engine life, whilst the smooth combustion and quiet operation of the original M-series engines are retained. Furthermore, the specific fuel consumption is, if anything, improved by the new system, a figure of 0-355 lb./b.h.p./hr. being quoted for the majority of the engine-speed range.
Still Multi-fuel
The HM-system does not reduce the multi-fuel properties either, and can be applied to any existing M.A.N. "M " engine, as evidenced by the four completely different types of " HM " power
unit on display at the Frankfurt Show this week, which show b.h.p. increases of up to 23 per cent compared with the " M " engines on which they are based.
The second important development which Dr. Meurer described to me last week concerns turbocharged M.A.N. engines. As is well known, blown automotive diesels are inclined to smoke if high torque is required at low crankshaft speeds, this being because of over-fuelling and proportionately low air output from the compressor. Throttling of the compressor can give the extra air flow required at low speeds, but this results in excessive air pressure at above the maximum-torque speed, so increasing peak pressures to an undesirable degree.
Peak-pressure Valve
Dr. Meurer has overcome this problem by making use of a principle not unknown in the diesel field in that a " peak-pressure governor valve" can be incorporated between the inlet and outlet sides of the exhaust turbine so that, before the air flow becomes excessive, the exhaust-gas flow causes the governor valve to open, thereby partially by-passing the blower.
In the case of the M.A.N. engines, twin exhaust manifolds are used, so the governor device consists of two butterfly-type valves on a single spindle, these being mounted off-centre whilst one end of the spindle carries a weighted arm. The use of twin valve flaps eliminates flutter which would otherwise be caused by the individual exhaust pulses.
To indicate the effect of the device, the power increase obtained in the case of the D2I46MT 9.6-litre engine is 10 per cent at 2,100 r.p.m., whilst the torque is increased from 550 lb. ft. at 1,400 r.p.m. to 620 lb. ft. at 1,300 r.p.m. The peak pressure, on the other hand, is reduced from 1,700 p.s.i. to 1,400 p.s.i. at 2,100 r.p.m., at which speed 230 b.h.p. is given.
Wankel Progress While on the subject of engines, Dr. Meurer was asked what progress M.A.N. were making with the Wankel nonreciprocating power unit which the company is licensed to produce by arrangement with NSU. Needless to say, M.A.N.'s line of development is concerned with a diesel-fuel version of this design and, as Dr. Meurer said, this engine " ;yes real problems to the diesel engineer" because of the high internal pressures involved.
It is inevitable that increasing the compression ratio must increase the size of the engine and its weight, whilst sealing of the engine was one of the biggest problems yet to be overcome. The doctor would say no more than this, beyond the fact that a certain amount of research progress was being made and that there was a long time to go before a commercially-successful diesel Wankel could be expected to appear. So it looks as though commercial-vehicle users will have to wait a considerable white for the first Wankel-powered diesel chassis, even assuming that one is ever produced.