Low-pressure Gas Deeply Studied
Page 25
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LAST week we reviewed very briefly a paper by Mr. E. A. C. Chamberlain, Ph.Dd., D,I.C., of the Gas Light and Coke Co., entitled "Some Problems Connected with the Simple Conveision of Petrol Vehicles to Operate on Low-pressure Town Gas," published by the Institute of Fuel, 30, Bearnham Gardens, London, S.W.5. As this paper contains certain information—the result of research for which Mr. Chamberlain is largely responsible—which, we believe, is of a a me what unique nature and constitutes valuable data, we present now a fuller synopsis of its contents,
Conversions of the type under consideration, the author explains, involve the minimum alterations, permit the use of either gas or petrol, and cost only a small sum. Such conditions militate against optimum performance, but it has been shown that simple conversions can be highly successful and that the restrictions imposed do not seriously affect the value of gas as an alternative fuel.
Town gas gives a lower power than petrol, and for this there are a number of explanations. His investigations in this connection have been made with gas having an analysis of CO2, 2.5; 02, 0.6; C5H6, 2.6; CO, 10.8; CH, 21.1; C2115, 0.7; H2, 54.4; 7.0, anehaving a heat value of 456 B.Th.I.J. net,
Running on gas, the fuel-air ratio is abOut 1 to 4.1 by volume. Thus, 20 per cent, of the mixture entering the cylinder is gas. With petrol, the fuel enters in the form of heavy vapour occupying less than 2 per cent, of the volume inspiratecl. In effect, therefore, a loss of volumetric .efficiency occurs with gas.
As a basis for comparison, Mr. Chamberlain gives the calorific values of theoretical fuel-air mixtures, for petrol and gas, respectively, as 95 and 85.2 B.Th.U. per cubic ft. Specific volume changes on combustion (i.e., volume of products to volume of mixture) are (petrol) 1.062 and (gas) 0.915. Mean specific heats of products of combustion, (0-2,200 degrees C.) are 0.02385 and 0.0244 B.Th.U. per cubic ft., for petrol and gas respectively. Petrol ini.s a latent heat of evaporation equivalent to 254 B.T1'.11. per lb. The specific gravities of petrol and gas nnixtures respectively are 1.04 and 0.89.
The effects of these factors are interdepe-Adent and only their general significance can be indicated individually. Most important is the 1.0.3 per cent, difference hi calorific value. Against gas also is its smaller volume change. On the other hand, the 1.5 per cent, specific heat difference means higher flame temperature and is in favour of gas. As the outcome, the author states that the theoretic pressure development on gas is less by 12.5 per cent.
Rich mixture results in an output rise. The explanation is that there is a larger number of molecules due to the existence of some CO and H in place of CO and water vapour. Mr. Chamberlain's tests have shown that maximum power was developed when the exhaust analysis gave 10 per cent. CO., and 1.4 per cent. CO.
Latent heat of evaporation improves volumetric efficiency by about 7 per cent., because, with iktrol, the charge is. cooled by about 25 degrees C. With gas, this is absent, Regarding specific gravity, the effect is likely to be inversely proportional to the square roots of the densities.
Thermal efficiences, under good conditions, can be higher with gas. Under ordinary conditions they are generally slightly lower. Compared with petrol figures of 22.6 per cent, for grade 1 and 20.9 per cent. for Pool, the nest figures for a Commer and an Austin respectively on gas were 17.9 and 20.4 efficiency per cent,
On the question of ignition advance, Mr. Chamberlain's experience is that best results are achieved with only a small advance on the. petrol setting. This is opposed to the usual view. It is possible, however, to run economically with an earlier timing and a weak niixture, but at the expense of power output, which cannot usually be afforded.
Some importance attaches to the sparking plugs. Overheating is common, Cures can usually be effected by setting the gaps ter 0.015 in. If special plugs are needed, Champion models LlOS and LA10, K.L.G. P7OX and Lodge IID, 14HB and 14 (Sintox) are recommended.
Requirements of a gas carburetter are to give a constant air-gas ratio over the whole range of operation; to cut off the gas when the engine is stationary; to give immediate response to throttle variations; to provide easy starting. steady idling, and freedom from stalling on disengaging the clutch; and to offer a low resistance to gas flow. In addition, the standard instrument, as produced, needs to be capable of being fitted to a wide range of engines of different types and sizes. Thus it must be adjustable for air-gas ratio.
Reviewing the matter of gas bags, the author refers to the need for preventing the fabric from blocking the outlet orifice as the hag becomes deflated, and stresses the importance of guarding against • chafing with consequent wear. He illustrates two practical arrangements—an open fronted crate with elastic tie cords, and the Walsh, with its device for positively controlling the bag by a parallel mechanism and for minimizing wind resistance. The advantages of this latter system are noted.
From his gas-petrol thermal-efficiency formula, Mr. Chamberlain, knowing the calorific value of petrol to be 1.505 therms per gallon, calculates that 315 cubic ft. of gas is equivalent to one gallon of petrol.
On the subject of gas-bag fabrics, the author is informative. They must come 'up to certain standards, he says, under the following series of tests:—Tensile strength, tearing, abrasion, stripping, weathering and permeability. He describes a device for investigating the last-named.
After weathering, a material should have a permeability not greater than 0.1 cubic ft. per 100 sq. ft. per hr., and the tensile strength should not have fallen below 40 lb. per sq. in.
The most satisfactory materials are those having a rubber layer sandwiched between cotton fabric. Calendered materials, in his opinion, are less satisfactory as they tend to peel and crack. Single proof materials can be satisfactory, given certain provisions. The proofing must be on the inside in contact with the gas.
Tests, as outlined, of (1) a rubber sandwich material, and (2) a single-ply calendered fabric gave the following results:--Weight per sq. yd., (1) 1.72 lb.; (2) 0.76 lb.; tensile strength after weathering (lb. per in. width): (I) 88; (2) 38; permeability in cubic ft. per 100 sq. ft. per hr,: (1) 0.052; (2) 1,43; reduction in tensile strength after abrasions, M 2.5 per cent.; (2) 23 per cent.; stripping time 1 in. by 1 in., (1) 105 sees,; (2) 5 secs.
Then Mr. Chamberlain tabulates some road-test results with a Morris van and an Austin car. Here are sonic extracts front the returns:—
Morris: acceleration "frorn 0-25 m.p.h.: petrol, 9.3 secs,; gas, 12.3 secs. Consumption: petrol 20 m.p.g.; gas, 19 miles per 250 cubic ft,
Austin : acceleration from 0.25 m.p.h.: petrol, 14 secs. ; gas, 14.6 secs.. Consumption: petrol, 28-30 m.p.g.; gas, 21-22 miles per 220 cubic Ct