On 25 March 1971, we detailed the bright but flawed idea that was the Pratt & Whitney-powered Lotus 56B

In a 1971 Formula 1 season dominated by Tyrrell, defending champion Lotus slumped to a disappointing fifth.

Both teams, along with every other customer team save for March, were using the era-defining Ford Cosworth DFV, a 3.0-litre V8 of relatively simple construction.

Look further down that year’s constructors’ championship table, however, and you’ll see that Lotus appears again, among the non-points-scoring teams, with Pratt & Whitney suffixed to its name.

The out-of-place appearance of the American aeronautical engineering company is the result of the Lotus 56B, another bright idea from the mind of Colin Chapman.

In 1967, a gas turbine car entered by oil company STP had come near to winning the Indianapolis 500. This powerplant was a modified verison of Pratt & Whitney’s Canada PT6, a turboprop engine well known to civil aviators.

In the simplest of terms, a turbine sucks air through a compressor, increasing its pressure. The air then enters the combustion chamber, where it is mixed with fuel so that the mixture ignites. This high-temperature gas then powers a turbine, which then turns the driveshaft. Jet engines are very similar in their operation.

In the 1968 race, an updated version of that engine was used in the specially designed Lotus 56. One of the three cars entered took a record-beating pole position, only for its fuel pump to fail right at the death with the car in the lead. The promise was evident.

Three years later, Autocar’s Edward Eves examined the B-spec version ahead of its grand prix debut.

“This turbine, in the 750bhp class, weighs as little as 2.9oz per horsepower. In comparison, the DFV weighs of the order of 12oz per horsepower,” we began.

“The snags with a turbine are that fuel consumption is doubled and that engine response is slow. This can be overcome in aircraft engines by very complicated design (twin spools or variable incidence compressor stators and computer-controlled fuel systems), but with a relatively simple engine the only way is to keep the throttle open and control the speed of the car with the brakes.

“For example, to have full power available at a certain corner, the driver will have to open up as he goes into it, having backed off long before he normally would. On a tight circuit like Brands Hatch, the car is likely to be at full throttle almost all the way round. Speed changes will be made on the brakes, working against the engine. Four-wheel drive is therefore essential, otherwise the braking imbalance between the front and rear wheels would be excessive.”

This also meant that the 56B’s driver would have to left-foot brake, which was almost unheard of at the time.

Autocar added: “Thus Lotus have to put up with the weight of four-wheel drive and an excessive fuel load in exchange for an engine which weighs half as much as the DFV and develops up to 500bhp at 45,000rpm.”

No gearbox or clutch were required, either. In comparison, the Tyrrell’s DFV was kicking out around 450bhp at just 10,800rpm.

“The 56B has a Ferguson apportioning centre differential driven from the turbine output shaft by a Morse Hy-Vo chain,” we continued.

“The drive from the centre differential is transmitted fore and aft along the left-hand side of the car by small-diameter solid drive shafts encased in guard tubes. The front and rear Lotus ZF locking differentials are developed versions of those used on the four-wheel-drive Cosworth-powered Formula 1 car and are housed in box extensions of the monocoque which, in this car, extends the full length to act as a cradle for the turbine aft of the car.

“Brakes are inboard with short solid drive shafts with GKN constant velocity joints to the wheels. In view of the extra load on the brakes in a turbine car, they have 10.5in-diameter ventilated discs with a good flow of air to the middle of the disc and extractors in the top panel of the nose.”

Gas turbines do not produce engine braking, so these enlarged units were vital. Still, they often overheated, with driver John Miles very lucky not to have been injured when they failed at Lotus’s Hethel test track.

“The chassis side sponsons are two simple boxes joined by the floor and by diaphragms front, rear and amidships. They hold the rupture-proof fuel bags containing 62 gallons of kerosene,” we wrote.

This was only enough for 160 miles – not a full grand prix distance. Therefore, we said, “the 56B will either have to be fast enough to gain enough time for a fuelling stop, be given bigger tanks or have its consumption improved if it is to be competitive with piston-engined cars.” Unfortunately, this issue was never resolved.

We continued: “Designed as a helicopter engine, the STN 6/76 two-shaft axial flow engine has been modified for this Formula 1 application. This has involved reducing the area of the combustion chamber nozzle, which feeds hot gas to the turbines, while two of the three stages of the axial-flow compressor have been removed.

“This takes into account the pressure ratio of the compressor, which is a function of the number of axial compressor stages with an allowance for the centrifugal compressor, which ‘bends’ the airflow outwards and into the annular combustion chamber. The hot gas expands through the chamber nozzle and drives the first turbine, which is on the same shaft as the compressor, and then goes on to drive the power turbine, which is on a separate shaft, connected through gearing and the Hy-Vo offset drive to the centre differential. The equivalent capacity of the engine is 2.95 litres.”

Evidently, such a system was rather complicated and, in view of the poor fuel economy and comparable horsepower with the DFV V8, what were the benefits of a gas turbine?

We explained: “It is light, so a great deal of fuel can be carried before it comes up to the starting line weight of conventional cars (heavier than the Lotus 72 but not excessive for a current F1 car).

“Maximum torque is developed when it is standing still on the starting line. Therefore, getaway is (theoretically) meteoric. Engine life, based on aircraft experience, is about 1000 hours, enough for several seasons without an overhaul.

“The saving in cost could therefore partly offset the high initial cost, which is estimated to be in the region of $75,000 (£58,600, or around £760,000 today).”

In the 56B’s first practice outing, at the hands of Emerson Fittipaldi around Brands Hatch in the wet, it was second only to Stewart’s Tyrrell. However, it floundered in the dry non-championship race.

The car’s potential for failing spectacularly and the unconventional driving style it engendered led to Fittipaldi later describing it as “the worst car he’d ever driven”, having “scared him every time he drove it”.

The car’s first two championship outings ended in non-powerplant-related retirements, while in its third and final time, it came home in eighth position. After this, Lotus cut its losses and abandoned the project. No serious turbine-utilising car has emerged since - until the new Ariel P40 arrives, that is

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Lotus Elise

Is the Lotus Elise still the last word in open-top British sports car fun?

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