AND THE NEXT BIT:
Germany is the only country in which any real attempt at enforcement is made and the Germans are very unimaginative, always tending to use the same parkplatz or “LKW Kontrolle” checks. Also, the two rips in six days’ schedule does not allow for my delays due to loading or unloading, customs, traffic congestion or weather, but n the absence of these the premium specification tractors are earning money at twice the rate of the small Volvos.
An old chestnut usually dragged out in iny discussion about premium specification -chides is that “power costs money” but his is a perfect example of the 1930s image mentality resulting from preoccupation with miles per gallon figures which, on their own, mean nothing. The technical press helps to perpetuate the this obsession with mpg by emphasizing mpg igures in road tests of lorries instead )f the true fuel consumption. In this 
ntext true fuel consumption is gallons per mile per ton per mile per hour multiplied by 100,000. To explain this, f irst dispense with miles per gallon, which is an unsatisfactory term for fuel consumption because the figure moves in the opposite sense to the parameter of which it is a measurement, and use gallons per mile.
The true fuel consumption is the fuel consumption relative to the gross weight if the vehicle, hence gallons per mile per ton, and to the average speed for he run so the final term is gallons per mile divided by the weight and by the average running speed and multiplied by 100,000 to give convenient figures.
A term like “per mile per ton per tile per hour multiplied by 100,000” s too ■■■■■■■■■■ for regular use, so it would be better to call it a “unit” and true fuel consumption is then measured in gallons per unit or GPU. In metric measurements this becomes litres per metric unit, the metric unit being per kilometre, per tonne, per kilometre per hour times 100,000. The figures are not much different, 1 GPU being 1.73 LPU.
Numerous road tests of lorries have shown that, for similar running times over the same route, total fuel consumption increases with gross weight, but true fuel consumption decreases with gross weight. This only proves that a heavy lorry is more economic than a light one, as is already generally accepted. At the same gross weight, total fuel consumption increases with average running speed, but true fuel consumption is not necessarily increased by higher engine power or power weight ratio. My table shows the results obtained in Commercial Motor road tests of 32-ton vehicles in 1971-72 (CM May 15) and comparison of the power weight ratio and true fuel consumption columns shows that, although one of the two vehicles with the lowest power weight ratio is the most economic, the other is less economic than two more powerful vehicles.
The Scania at 8.6 bhp /ton has the same true fuel consumption as the Scammell at 6.5 bhp /ton and less than the AEC at 6.4 bhp /ton, while the DAF with the second highest power weight ratio has the second lowest true fuel consumption. Thus there is no correlation between true fuel consumption and power /weight ratio, as expected, and the most significant difference in performance is from one make to another. This is thought to be due to variation in transmission efficiency.
In general terms the same power is needed to move a given load at a given speed irrespective of whether the maximum power of the engine in use is 220 or 340 bhp. This assumes other things being equal such as trailers with a 4 x 2.5 metres cross-section in each case and the vehicles having the correct gear ratios. Perusal of performance curves will show that overall fuel consumption will be the same in each case. In fact specific fuel consumption of the more powerful engine is sometimes lower, balancing out the higher power which the heavier-duty transmission absorbs.
In the case of the Rolls-Royce Eagle 220 and 340 engines when both are producing 220 bhp, the specific fuel consumptions are 0.390 and 0.3581b /bhp / hour respectively giving fuel consumptions of 9.4gal/hour for the 340 bhp engine and 10.2gal/hour for the 220 bhp engine. These figures would be reflected on motorway running, provided the more powerful vehicle was not under-geared, and even at the same engine speed the turbocharged engine is more economical as the part load specific fuel consumption, when producing 220 horsepower at 2100 rpm is 0.375 lb /bhp /hour.
Fact, of course, does not bear out theory, because the driver of the more powerful vehicle will use his extra power to reduce his running time, thus increasing his fuel consumption. But time is money and the time saved could be particularly valuable if it enabled a run to be completed in one day instead of running into a second day. There is more than one operator whose drivers regularly work a 14-hour day at present on day runs. This consists of up to 1+ hours refuelling and swopping trailers because no yard ’ shunter is employed, trailer winding handles and number plates can never be found and usually both vehicles are boxed in, followed by a 12±hour “spreadover” to complete the run in a day with underpowered tractive units. If there is any delay extending the run beyond 124 hours, a “breakdown” is booked on the log sheet which is quite plausible considering the state of the units used by most operators of this type. These people will have to use premium specification tractive units to stay in business when tachographs become mandatory unless they can think of ways to fiddle the tachograph records.
I’ll keep hunting too. 
By the time I get to 5000 posts 
I’ll be ready to start book 2!! 
Robert
