BOOSTING-OUT THE PRC UCER-GAS POWER LOSS
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BECAUSE of its lower calorific value, producer gas, when used instead of petrol. as a fuel for road vehicles,' gives a power output that is, more or less, proportionately smaller. A drop at least of 40 per cent, in b.h.p. results from a straight change-over, provided that no measures are taken to improve matters in other ways.
Respective c.v.s for petrol and solid fuel are, approximately, 14,000 B.Th.U. and 20,000 B.Th.U., so the difference is 30 per cent.,: but it is the mixture as drawn into the cylinders that counts, and here the superiority of the liquid fuel is in 'greater evidence. Furthermore, the gas is generated at a high temperature and is probably still quite warm when it reaches the induction pipe, with the result that its density is lower, whereas petrol, in evaporating, gets cooler and thus the combustible gaseous mixture is of higher density than it would be at atmospheric temperature. In addition, there may •be resistance in the filters and pipe line causing further pressure drop.
An engine's thermal efficiency can be increased by raising the compression ratio. That is one of the reasons why an oiler goes more miles to the gallon than a petrol vehicle. A limit, however, to compression is set by detonation. With a commercial petrol, 6 to 1 is about as high as is practicable, because of knocking and pinking. Gas is less prone to detonation, so higher compression ratios can be used-8 to 1 is usual—and advantage is taken of this fact to compensate, partly, for the power loss.
Accordingly, we see that the inherent inferiority, due to calorific value, can be lessened—at least to a moderate extent—in two ways, namely, by improving effective volumetric efficiency and by raising the compression ratio to afford better thermal efficiency. Clearly, the former helps the latter; a cylinder well charged with gas will ' produce a higher pressure, when the piston reaches 'the top of the stroke, than one of which the contents, at the end of the induction stroke, are rarefied.
With this groundwork distinct in our minds, we can comprehend how supercharging helps a lot. The function of the blower is to fill the cylinders with gas at more than atmospheric pressure, so that, without longer pistons or machined-down cylinder heads, the actual pressure at the moment of ignition is equivalent to that in an engine of higher compression ratio, whilst, in addition, there is more mixture to burn. Theoretica,lly, then, a power output approaching or, indeed, exceeding that on petrol might seemingly be anticipated from an engine power-charged with producer gas, and thus there appears to be justification for claims made in respect of a lorry that has been equipped with a blower and is running on producer gas, the performance of which we recently observed. Moreover, our own impressions, formed in the courseof a 62-mile run in actual service, tend to corroborate these claims.
With admirable enterprise, Messrs. Harris Garage, 44, Brewery Road, London, N.7, have been pioneering this movement. This firm are responsible for the trials under consideration, and are associated with the operation of this vehicle. It is a Diamond T, and is used with a Cowan trailer producer-gas pla..t, pressure charging being effected by an Arnott blower. A brief description of the outfit was included in our issue dated February 27.
Rated at 31.54 h.p., the engine of this lorry has six cylinders measuring 82 mm, by -108 mm. (4,51 litres) and develops on petrol (unsupercharged) 78 b.h.p. at 2,800 r.p.m. On the outward journey of our trial run the lorry carried a pay-load of about 31 tons . scaling all-up about 6 tons 19 cwt, Whilst we were unable to obtain any definite figures for performance (other than weight of fuel consumed and miles traversed) or to make comparisons with the vehicle's running on petrol, because of circumstances beyond our control, we can state that we were much impressed by the speed, hill-climbing, flexibility and liveliness demonstrated.
There were occasions when we were able to overtake up hill vehicles of apparently the same class and running under seemingly like conditions, but, of course, on liquid fuel. Furthermore, we were accompanied throughout the test by a Morris car, also equipped with a Cowan plant, but in this case built in, and there appeared to be very little between the respective performances of the two vehicles except in respect of maximum speed.
For the -first 36 miles full load was carried; for the next 8 miles we had about half load on board, and the return journey of 18. miles was made with no load. Over the total distance of 62 miles, 50 lb. of Rexo fuel were consumed, which represents the excellent figure of 0.8 lb. per mile or 1.2 m.p.lb, Here are a few rough observations noted on route ; Time from Brewery Road to Archway Tavern (2 miles), 51 mins. Average speed up Archway Road, 15 m.p.h, in second and third gears. Time to reach Tally Ho Corner (6 miles), 20 mins. Time to climb Barnet Hill (1 mile, from railway bridge to church), 3 mins. 5 secs.. using second and third gears. Minimum speed up Brookman's Park Hill 20 m.p.h. using third gear. No allowance has been made fur traffic, balks which were frequent.
At the outset only two minutes elapsed between lighting the fire and starting away on good gas; however, 1 lb. of charcoal was put in to accelerate the initial combustion.
An excellent demonstration of flexibility Was afforded by the manner in which we negotiated Hatfield. Top gear was used exclusively and, without faltering or any sign of complaint, the vehicle threaded its way slowly through the town, up the quite appreciable gradient and round the sharp corner on to the Hertford road.
On various occasions during the run speeds in excess of 40 m.p.h. were attained, and throughout the day no trouble of any sort was experienced. The only delay that occurred was in starting up on petrol after a long pause for unloading. Once on petrol gas was quickly generated. Restarts after short stops could be made on gas. On the return journey on strange. by-roads the car lost touch with the lorry, so its driver, thinking he was ahead.
waited for about 15. minutes. On arriving home, he found the Diamond T had got in 20 minutes before him.
We understand that the lorry engine has a compression ratio of 5.8 to 1 and that the supercharger, which runs at I I, engine speed, gives a maximum induction-pipe pressure of 7 lb. per sq. in. Thus, the compression pressure, with the blower, should be 122 lb. per sq. in., which is equivalent td raising the ratio to 8.7 to 1. These figures, however, must on no account be regarded as more than a rough guide.