TURBOCHARGED IN-LINE ENGINES
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-VOLVO CLAIMS By P. A. C. Brockington ENGINEERS of the Volvo company of Gothenburg, Sweden, have given their views on the V versus in-line controversy and have provided me with details of the measures taken to reduce noise levels.
Mr. Ake Larborn, chief engineer and manager of Volvo development laboratories, has sent the following replies to CM's questions: 1. In comparison with naturally-aspirated engines the V engine is likely to produce a higher output for a given weight and space because of its compactness and higher rotational speed.
2. In turbocharged form the advantage of the V engine is less evident and this will apply at least as long as the V engine has not proved itself as suitable for anything above mild supercharge, that is 20/25 per cent.
3. Compared to any naturally-aspirated engine the turbocharged engine shows a significant reduction in fuel consumption because of the lower friction horse power connected with the smaller swept volume engine. If the naturally-aspirated engine also has a higher operational speed, this difference can be most important. Remember that we have to burn fuel to overcome friction all the time, virtually independently of the loading of the engine. In other words at the average load of say 40 per cent of the maximum output, the fuel consumption is considerably lower with the turbocharged smaller-volume engine.
4. The V engine is considerably more expensive to produce even compared to an in-line engine, where a comparable output has been achieved with a fairly high supercharge (50/75 per cent).
5. This cost difference is more striking when, as for example with Volvo, several different sized six-cylinder engines are pro
duced with virtually the same production equipment, transfer lines, assembly line etc.
6. The in-line engine is more universally adaptable to different vehicle configurations. It can be used mounted at the front, transversely or longitudinally at the rear, under the floor and so on with few modifications.
7. We are confident that in-line turbocharged engines can amply satisfy any European road vehicle power requirements for the next decade.
The main factors contributing to the low noise level in the cab of the Volvo FB 88 are: 1) Turbocharging of the engine; 2) measures for reduction of noise radiation from engine surfaces; 3) reduction of fan speed by using Visco fan; 4) carefully designed intake and exhaust-noise silencers; 5) filt cab with permanently closed engine cover that obviates any noise leakage to the cab interior; 6) high engine torque permitting extensive running in high gears at low to medium engine speed.
The first four points also contribute to a low external noise level when noise measurements are made in accordance with legislative requirements.
The reduced noise level when turbocharging the engine is no doubt connected with the increased temperature of the air charge to the cylinders, which reduces ignition lag and hence combustion roughness. When we measure the total noise level of the vehicle according to the ISO procedure, we find a reduction of 2/3 dBA when turbocharging is introduced, in spite of the increased rating. A large part of the total noise emission from a vehicle can be traced to noise radiating from surfaces on the engine. The source is of course combustion noise and mechanical noise from timing gears, pump drive and so on. During the development of this engine particular studies were made with the engine running in non reverberation surroundings. Noise reductions were achieved by redesign of the front timing gear cover and the crankcase walls. The inherently sturdy construction of an engine designed for high output is also helpful.
The fan noise has a high pitch and is therefore subjectively very noticeable at high speed. The introduction of a Visco fan has virtually eliminated this noise source. The slip of the fan is thermostatically controlled by the temperature of the air on the engine side of the radiator and the normal fan speed is very low. The ISO noise reduction attributed to the Visco fan is between 1 and 2 dBA. Noise reduction is more noticeable when the fan is not working at full speed.
The development of suitable intake and exhaust silencers is an obvious step but this is considerably easier for a turbocharged engine because of the damping effect of the compressor and turbine wheels.
Even small leakage areas between engine compartment and cab can cause a high degree of noise transmission. In the Volvo tilt cab the entire floor together with the engine hood is a welded and sealed unit. Only the steering column, throttle linkage and gear change lever require an opening in this surface and they are easily sealed with rubber diaphragms.
These factors added together enable a total noise level, measured to ISO-362 and ISO419 standards, of around 85 dBA.