Search for Improved Blowers
Page 60
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R. TRYHORN said that it was iVi possible to provide more oxygen for combustion, by pressure charging an oil engine and so making it run in an artificially dense atmosphere. The quantity of nitrogen also increased, so that the temperature rise of the charge during combustion was no different from that of the naturally aspirated engine, and was still well below that for a petrol engine. Consequently, there was no rapid deterioration in engine reliability and life when it was pressure-charged.
It was important not to ,confuse charge pressure with density. The job of the pressure charger was to increase the charge density, not its pressure. The pressure would be raised, but the pressure in itself was of no value and was not even a measure of the charge density. The difference between the two was the charge temperature, which depended upon the efficiency of the compression process.
Effect of Efficiency
Any charger which had an efficiency of less than 25 per cent, was useless as a means for supplying more oxygen to the cylinder, whatever pressure was used, even if its driving power was obtained free. Conditions improved fairly rapidly as the charge efficiency increased above 25 per cent. The 50per-cent. line was not far from what would be expected for the efficiency of a Roots blower of the size used for automotive applications up to the pressure ratio of 1.6 to 1.
At this efficiency a pressure of 7.1 p.s.i. was needed to give an increase of 20 per cent, in oxygen to the engine. The importance of high charger efficiency was obvious when it was considered that a pressure of only 5.1 p.s.i. was required to supply the same amount of oxygen if the charger was 75 per cent, efficient, and only 3 p.s.i. if the air was completely cooled.
Any cooling of the ingoing air increased the charge density. It was, therefore, equivalent in some ways to c20
increasing the charger efficiency. If the object of pressure charging was merely looked upon as a means for raising the charge density, a simple yardstick to evaluate the effect of cooling up to about 60 per cent. was that charge cooling of 10 per cent, was equivalent to approximately 5 per cent. increase in charger efficiency. Over 60 per cent. cooling -gave a more than proportionate improvement.
The Roots blower was often referred to as a simple displacing machine. At very low speeds this might be so, but at useful operating speeds it was far from the truth. Pressure waves were built up and these had harmful secondary effects.
They made the blower performance very sensitive to the geometry of the air delivery system and they constituted the working load on the mechanism. Thus the torque and bending loads on the rotors were caused by pressures peaking up to at least double the delivery pressure and fluctuating at high frequency.
Nevertheless, the Roots blower could • show a reasonable return in terms ofextra power from the engine for the associated increase in cost. However, there was plenty of incentive to find better types for this purpose.
There appeared to be two distinct pressure-charging requirements. The first was to increase engine output by 15-30 per cent. as cheaply as possible. This meant the use of low-cost blowers of the order of, say, ft per extra horse power obtained, with a minimum of modification to the engine and its components from the naturally aspirated version. •
The second need was to obtain an increase of at least 50 per cent, in engine torque from what, might loosely be called a permanently pressure-charged engine; this engine might not be economic in a naturally aspirated form.
Increases of torque of this order required charge pressures of 7-14 p.s.i., which were beyond the efficient operat