Stalin said that war was the locomotive of history, and it is certainly the locomotive of automotive engineering. The first viable turbocharger (at the time referred to an exhaust-driven supercharger) was run on a Bristol Pegasus radial aero engine, which enabled a height record of 32,000 feet to be set in 1923. At the time, and indeed until the late 1930s, the standard method of supercharging reciprocating engines was using a mechanical, usually crankshaft-driven, supercharger - the 'Blower' Bentleys of the 1930s are still famous for the great polished Rootes supercharger sticking out from under the radiator. The Mercedes Kompressors had an engine designed by Ferdinand Porsche and featured a supercharger connected to the crankshaft by a dog clutch so that, as the driver floored the throttle, the supercharger wound up and began pressurisation.
The crank-driven supercharger persisted in aero-engines in various incarnations. Rolls-Royce Merlins and Griffons were fitted with variously single-stage and two-stage, single-speed and two-speed superchargers driven from the free end of the crankshaft and, in the case of the Merlin, in a 'suck-through' installation with a three-choke SU carb and air-water aftercooler. The contemporary German vee-12 aero-engine, the Daimler-Benz DB601, had Bosch direct fuel injection instead of the Merlin's carburettor to give better negative-G fuelling.
Rolls-Royce Griffon with two-stage, two-speed centrifugal supercharger. This is an engine from a Shackleton MR3 aircraft, with contra-rotating propellers - note the concentric propeller shafts
In the US, General Electric had used its experience with steam turbines for power generation to design aero-engine turbos (now called turbo-superchargers) for engines like the Wright R-1830 Cyclone fitted to B-17s and B-24s. It was also famously fitted to the Allison V-1710s installed in P-38 Lightnings, and were (due to their secrecy) were removed from the three P-38s sent to the RAF in 1941 for testing - without the turbochargers, the P-38 was an appallingly sluggish aircraft, and the RAF didn't want them.
The aero-engines made in 1945 were, by and large, evolutions of 1939 designs. The Merlin was developed, going from about 1050hp in 1939 to 2100hp in 1945, but it remained fundamentally the same engine. In the US, engines mainly got bigger, and the R-1830 begat the R-3350 Duplex Cyclone (the numbers refer to the swept volume in cubic inches - 55 litres, in this case). which was fitted to such wonderful aircraft as the Boeing B-29 and the Lockheed L-1049 Super Constellation. Pratt and Whitney built the R-4360 Wasp Major or 'Corncob' 28-cylinder four row radial which was fitted to Boeing B-50s and the vast Convair B-36. It was such a complex engine that it took five hours to complete a B-36's starting procedure, and six hours to shut the engines down safely.
Wright R-3350. Their magnesium crankcases caused a propensity to catch fire and led to them being known as 'Wrong R-3350s' or 'Flamethrowers'
In Germany, however, some odd requirements from the Reichsluftfahrtministerum, the Air Ministry, led to some weird and wonderful ideas. The Daimler-Benz HZ-Anlage was intended for high altitude (above 50,000 feet) bomber aircraft, and consisted of a fuselage-mounted DB605 driving a huge supercharger, delivering air to the two wing-mounted DB603s. Slightly more sensible ideas involved putting a turbo and a supercharger in series, thus providing more boost across all altitudes and without throttle lag.
Daimler-Benz DB605, fitted to Bf109F fighters. A centrifugal supercharger is mounted on the port side of the crankcase.
Desperation towards the later stages of the war (deepened after the first jet aircraft proved frighteningly unreliable and, in the case of the ME163, doing more harm than good) led to investigations into methods of boosting piston engine outputs beyond the usual emergency output for short periods in the absence of high octane petrol which would allow excessive boost to be used - sprinting to catch a bomber formation, or trying to evade enemy fighters, for example. Military aircraft always have short-term power ratings for emergencies, 'war emergency rating' which will kill the engine in five minutes without throttling back, or else 'max mil' or maximum military power which is a higher output that maximum ordinary power, mainly because the Queen pays for the parts. Luftwaffe, BMW and Daimler-Benz engineers came up with two main solutions, MW50 and GM1.
MW50 involved spraying methanol and water in equal parts (hence the name) into the eye of the supercharger. This had two main effects in that it cooled the fuel/air mixture, thus increasing the charge weight, and the methanol acted as an additional fuel to complement the petrol.
GM1 was gaseous nitrous oxide and extra fuel injected into the inlet tract downstream of the blower. Again, this cooled the charge, increasing its density. The nitrous oxide also decomposed in the combustion chambers to give extra oxygen, permitting the fuel injected at the same time to be burnt. GM1 became known as 'ha-ha' for obvious reasons and, when used with MW50, would produce very significant power gains for no more than one or two minutes at a time. The Luftwaffe's greatest problem was that the fuel available to them was of dreadful quality - to help Tempests and P-47s catch V1 flying bombs in 1944, 750hp gains could be had by increasing the war emergency boost to ludicrous amounts (60psi), and using 150 octane aviation fuel called 'purple passion.'
After the war, MW50 and GM1 were more or less forgotten about. As jet engines were developed, the piston engine was relegated to smaller, low performance aircraft, although water injection was fitted to engines like the GE J-47 in Boeing B-47s to boost takeoff power but it too died its death after a while
It was not until the early 1960s that turbocharging and so on began to rise in prominence again, this time due to the American motor industry. The Oldsmobile Jetfire of 1962 had a Garrett turbo fitted to its 215cu in vee-8 in a suck-though configuration and developed 215hp. When the boost pressure of an engine is increased, so is the risk of detonation and holed pistons; to counter this, the compression must either be lower than an equivalent normally-aspirated engine, higher-octane fuel should be used or the charge should be cooled - Oldsmobile engineers, remembering German experiments during 1944, used methyl alcohol-water injection. Pre-mixed, it was sold through Olds dealers and branded as 'turbo rocket fuel.' The Jetfire beat the Chevrolet Corvair to the market by a month; this had a rear mounted flat six, all-independent suspension and was famously available with either four carburettors or a turbo. Both cars were reasonably unsuccessful, due mainly to the low price of petrol in the US (why make an efficient, tuned and expensive three-litre engine when an inefficient seven-litre motor will do the job cheaper?) The Jetfire engine, without its turbo and 'turbo rocket fuel,' later became the Rover vee-8.
The Corvair engine. Note the clearly visible suck-through arrangement
On the drag strip, tuners found that big block vee-8s could be tuned to bursting by fitting immense Rootes blowers and using methanol as the fuel - 'top fuel' in the argot. Where it was impossible to fit a blower, racers used nitrous oxide injection for short-term and significant power gains. Nitrous oxide isn't a fuel, and is thus road legal. However, additional fuel is injected through the nozzles with the nitrous to prevent the mixture leaning off and destroying the engine and, assuming that the nozzles are correctly jetted, a sensible installation doesn't shorten the life of the engine. Because nitrous is quite expensive, it is only used sparingly - systems are generally wired with a throttle switch, so that injection only takes place on wide-open throttle.
Methanol in action. Eight-litre vee-8s develop up to 2,500hp for very short periods; backfires can cause the blower (which absorbs about 400hp at full speed) to be blown off the engine. It is also the very devil to extinguish, as foam absorbs methanol to create highly flammable bubble bath
The pinnacle of forced induction technology was probably the turbocharged Formula One cars of the late 1970s and 1980s. Under the rules of the time, normally-aspirated engines could be up to 3,000cc, whereas engines with forced induction were only permitted 1,500cc displacement; this differential was established in 1962, when supercharger technology was fairly simple. However, Renault built an experimental 120-degree vee-6 with one and later two huge turbos and, assuming that the engine held together, eventually won races. Ferrari followed with a very similar configuration and were followed by TAG and BMW who chose twin-turbo straight-fours. The Brabham-BMW BT52 driven by Nelson Piquet was the first turbo car to win the F1 championship, and BMW went on to power ART, Arrows and Benetton in addition to Brabham before getting out of F1 when turbocharging was banned. By the end of the turbo era, 'qualifying boost' would produce around 1,400hp whereas a competitive 3,000cc 'atmo' engine would do well to produce 550hp. Ferrari fitted additional fuel injectors upstream of the turbines - the fuel would ignite, and the turbines would spool up far quicker, reducing lag. When asked how much power the M12 engine fitted to the Benetton B186 developed, Paul Rosche said 'it has to be more than 1,400 - the engine brake only measures up to 1,280...'
The Ferrari 126 C3. Note the wastegate actuator (silver cylinder on top of the engine), one of the two intercoolers (angled downwards in front of the engine) and the two ram pipes, one for each turbo.
After the Corvair and Jetfire, supercharged road cars had to wait until the early 1970s to return to sale. BMW built the 2002 Turbo (the bottom end of which became the M12 Formula 1 engine) mainly because, without the turbo, improving the performance of the 2002 Tii would have required fitting a bigger engine which would have upset the balance of the car. This formula was retained by many other manufacturers, almost always European or Japanese, as large displacement engines don't fit small cars - even a (relatively) small American muscle car like the AMC AMX 390 or the Plymouth Hemi 'Cuda are, by comparison with a European sports saloon, very large.
The BMW 2002 Turbo's four-cylinder engine was fitted with Kugelfischer mechanical injection and a KKK turbo. Unfortunately, the smallest KKK blower was far too big for a small petrol engine and it endowed the car with enormous turbo lag (the time needed for the turbocharger to spool up and begin supplying boost to the engine - a small blower has less rotational inertia than a larger one and is able to spool up faster). Unfortunately, the '02 Turbo was remarkably thirsty and it was launched at the beginning of the fuel crisis.
BMW 2002 Turbo, all of which were fitted with the discreet body kit by the factory
In contrast to the technical sophistication of the '02 Turbo, the first British turbocharged production car was a model of blacksmith engineering. The TVR 3000M was fitted as standard with a mildly-tuned Ford 'Essex' three-litre vee-6, breathing through a downdraft Weber carb. The 3000M Turbo was modified by Broadspeed Engineering, who used tubular exhaust manifolds going forward to a Holset turbo which fed the standard Weber. However, carburettors have to be built specially to withstand blow-through supercharging - if the air pressure is greater than the fuel pressure, fuel will be forced back through the jets to the fuel tank, and Broadspeed's solution was to put the carb in a sealed cast alloy box which was then pressurised so that the relative pressure between the air and fuel sides of the carb was maintained. They also stripped, balanced and blueprinted every engine as a hedge against them blowing up (none did). TVR only made 63 M series Turbos.
The 1977 Saab 99 Turbo probably marks the coming of age of the turbocharged road car. It was reliable, civilised, very discreet and safe. The Triumph derived slant-four had the simple and effective Bosch K-Jetronic mechanical fuel injection and a Garrett T3 turbo, which was designed for car use. Garrett fitted the T3 with a wastegate, which 'wastes' excess exhaust gas pressure past the turbine into the exhaust system which presents the immense advantage of greatly reducing turbo lag. The only difference between the Saab 99 Turbo and a current car like the Audi TT is that the Audi is fitted with electronic engine management and intercoolers and are essentially no more reliable or driveable than a 1977 Saab.
As the charge air is compressed in a supercharger it is heated by the same phenomenon that makes Diesel engines work. Hot charge air leads to a loss of efficiency and intercoolers (fundamentally air-air heat exchangers) which were originally fitted to truck engines began to be fitted upstream of the turbo - in the picture of the Ferrari 126 above, the intercoolers are obvious and actually larger than the coolant radiators. On many current turbocharged cars, the intercoolers are equally obvious: the Mitsubishi Lancer Evolutions have the intercooler on show under the vestigial radiator grille, while Subaru Impreza Turbos have the intercooler mounted horizontally above the boxer-four engine and supplied by air through the distinctive bonnet scoop. In the latter case, the horizontal position of the intercooler is efficient, unlike that fitted to the Nissan Sunny GTiR, which is atop the (hot) engine, manifolds and turbo - when the Sunny was being unsuccessfully rallied, the engine lost about 60bhp as the engine bay warmed up and the team referred to the intercooler as the 'interwarmer.'
One component not fitted to truck and bus turbo diesel engines is the dump valve. Fitted upstream of the intercooler, the dump valve works by dumping excess boost in situations where the turbo provides more pressure in the inlet tract than the engine requires, for example on the over-run where the turbo shaft is still spinning from the power being applied under acceleration. This has the benefit of avoiding compressor stalling and speeds response, as the turbo is kept spinning. Many production cars are fitted with recirculating dump valves, where the excess pressure is dumped into the inlet tract but atmospheric dump valves are often fitted to tuned engines - atmospheric valves dump to atmosphere and produce a very distinctive 'whoosh' as they operate.
In recent years, mechanical supercharging has made something of a comeback. Lancia fitted Volumex blowers to its Trevi coupe in the late 1970s (all of which dissolved in the great tradition) and Volkswagen installed 'G-ladern' to Polos and Golfs in the late 1980s. The Volkswagen G-lader used a pair of meshing volute chambers to pressurise the inlet tract and were physically much smaller than a conventional Rootes blower. Neither the G40 Polo nor the G60 Golf were immense sales successes.
Jaguar took a brave step in 1993 with the XJ-R. Based on the XJ-6, it featured an Eaton M40 Rootes blower fitted to the 4.0 litre AJ-16 straight-6 engine with uprated braking and suspension derived from the XJ-12 and was something of a success. It delivered 345bhp, the same as the 6.0 litre vee-12 XJ-12, but was cheaper, lighter and easier to maintain, and a sleeved-down 3.2-litre version of the XJ-R was fitted to the Aston Martin DB-7; for both Jaguar and Aston, it represented their first blown road car. The straight-six, XJ-40 based XJ-R gave way to the X300 based vee-8 XJ-R, one of which gave service as the fire tender for the Thrust SSC land speed record team. The XJ-R engine is currently available in the XJ-R saloon, XK-R grand tourer and Daimler Super V8 luxury saloon.
Factual inaccuracies? Click here.
Back to Home Page