Main engineering developments adopted by Thornycroft during the period 1927 to 1932 included six-cylinder engines, diesel engines, vacuum servo-assisted brakes and all-wheel braking. The first three of these developments are looked at below.
In 1928, Thornycroft relied on four-cylinder engines for powering its entire range of lorries. The most powerful engine produced by the firm at that time was the 6,970cc MB/4 giving a modest 60bhp@1,800rpm, and the maximum load capacity of the model range was 7 tons. However, customers started demanding lorries able to carry greater loads. More weight needed more power, which could only sensibly come from a larger engine rather than by uprating an existing one. Instead of increasing the swept volume of the seven-litre four-cylinder MB/4, Thornycroft opted for six cylinders and in 1929 introduced the 7,763cc WB/6 (78bhp@1,500rpm). This engine powered the JC 10 ton (10,160kg) RSW, which, in capacity terms, was Thornycroft’s largest lorry in 1929.
Introduced in 1929, the JC 10 tonner had a six-cylinder petrol engine and Westinghouse servo braking. Shown before the body was installed.
However, the JC did not last long and a larger machine, the QC RSW, was introduced in 1930 capable of carrying 12 tons (12,192kg), powered by the mighty 11,330cc NC/6 six-cylinder engine giving 104bhp@1,700rpm. The WB/6 engine which had powered the now-defunct 10 ton (10,160kg) JC was relegated to lighter lorries (e.g. 6 tons/6,096kg) which gained increased performance as a result. Thornycroft decided that the benefits of six-cylinder engines, such as smoother running, justified their wider use and ‘sixes’ spread down the capacity range, the smallest lorries to be powered by ‘sixes’ were both the A7 and A14 versions of the four-wheel Speedy 2.5 tonner (2,540kg) in 1931. Thus, four and six-cylinder engines were used to power Thornycroft’s range of lorries and, on some models, customers were offered a choice of either.
The comparison in power outputs between the 6,970cc MB/4 and the 7,763cc WB/6 begs the question as to how the WB/6 (78bhp@1,500rpm), with only 11 per cent more swept volume than the MB/4 (60bhp@1,800rpm), could develop 30 per cent more power at much lower rpm. It is possible that the MB4’s rpm should read 1,600 in line with its similarly-powered and sized BB/4 predecessor, while the WB/6’s rpm should read 1,800, a speed at which it was said to be safe. In this case, the WB/6 could develop 75bhp when scaled up from the MB/4’s 60bhp, which is near enough the published figure of 78bhp. Using a similar line of reasoning to justify the 11,330cc NC/6’s claimed output, then scaling up from the MB/4 (taken as 60bhp@1,600rpm) gives an output for the NC/6 of 104bhp@1,700rpm, which matches the published figure.
Diesel engines were well-established in marine and land applications by the time they were offered in road vehicles, for which diesels were potentially attractive thanks to their fuel economy. Mercedes Benz started the diesel ball rolling in the UK when it exhibited a 5 ton (5,080kg) diesel lorry at the 1927 Commercial Motor Show, and the following year the firm started offering the vehicle in the UK, the first diesel lorry to be put on the British market. Also, in 1928, a German diesel lorry engine, the Cologne-built Deutz, was given press coverage in the UK.
Announced late in 1932, the 6.5 ton Taurus was offered with diesel and petrol engines.
Interest in diesel-engined commercial vehicles grew and in late 1929 they were listed for the UK market by Berna (Swiss), Mercedes Benz (German) and Saurer (Swiss). Also, as a result of tests with diesel engines, the London General Omnibus Company ordered 27 diesel buses for experimental use, most of the engines for which would be built by the British firm AEC and the remainder overseas. By the end of 1930 manufacturers listing diesel commercial vehicles in the UK included AEC (British), Berna (Swiss), Laffly (French), Mercedes Benz (German), Pagefield (British), Peerless (British of US origin) and Saurer (Swiss). Although still without a diesel lorry to sell, Thornycroft announced that it had been running experimental diesel vehicles for some time and could supply diesel vehicles as soon as demand arose.
Thornycroft displayed its first vehicle diesel engine in 1931 at an exhibition of diesel-engined vehicles in Manchester. Resembling Thornycroft’s NC6 petrol engine, this 10,741cc ‘six’ developed 90hp at 1,800rpm and was offered in the Type QD Colossus RSW 10-11 tonner (10,160-11,176kg). Other British exhibitors at Manchester’s diesel exhibition included AEC, Blackstone, Crossley, Gardner and Leyland. Despite joining the diesel ‘club’ Thornycroft was cautious about diesel vehicles, Sir John I Thornycroft said that it was no use obtaining great improvements in fuel consumption if the savings were offset by increased maintenance charges. He also said that forecasts of widespread use of diesels neglected the effects which a special form of taxation might have on diesel fuel.
In late 1932 another Thornycroft vehicle diesel engine was announced, reportedly entirely different to the firm’s first vehicle diesel which was little more than an adapted petrol engine. Two versions of the new diesel engine were offered, the 92bhp four-cylinder CIND4 and 147bhp six-cylinder CIND6, both outputs being given at 1,800rpm. Swept volumes were 7,500cc, and 11,250cc, respectively, and both versions of the engine gave no less than 56 per cent more bhp/litre than Thornycroft’s original NC6-based diesel. A Taurus chassis fitted with a CIND4 went into production as the JD/CI-ND4, alongside petrol-engined versions of the Taurus. Thornycroft was now a mainstream diesel lorry manufacturer.
Vacuum servo-assisted brakes
Servo-assisted braking systems fitted to Thornycroft lorries amplified the force applied by the driver’s foot on the brake pedal, relieving the driver of having to apply a large pedal force to obtain effective braking. Servo effect was proportional to brake pedal force.
Thornycroft standardised on the Westinghouse vacuum-operated servo-assistance system and first fitted it to at least one of its models in 1929. The use of servo-assisted braking spread throughout the Thornycroft lorry range over the next three years, and by the end of 1932 all the firm’s lorries were listed with it.