Any doofus knows that the supercharger increases the mass flow of air into the engine - a simple principle of increasing the rate at which fuel can be burned in that air. But supercharging was, for many decades, problematic, mainly because of the considerable thermal problems associated with forced induction. As the mass flow of air into and through the engine is increased, the heat flow is increased in at least equal measure.
Sir Dugald Clerk
Even though it had its problems, supercharging can be traced back to 1901, when two-stroke pioneer Sir Dugald Clerk advocated the use of supercharging to create an artificially dense atmosphere from which the engine could breathe, the extra air being meant to control the maximum temperature reached in the combustion phase. In the process, he found a 6% power increase and so the search for supercharged power began.
In 1902, Louis Renault
had ideas for it in France; in 1909, Lee Chadwick made it work effectively on competition cars in America; and in the same year Biichi in Switzerland hit on the idea of a supercharger driven by an exhaust
turbine for diesel engines. After that, developments came rapidly, first on aero engines during World War 1 (because the supercharger or turbocharger could compensate for the low atmospheric density at high altitudes) and then in the 1920s for sports and racing cars, most of the pioneering being done by Mercedes and Fiat.
From then on, supercharing for racing cars never looked back, until in 1951
Aurelio Lampredi turned the tables on the unbeatable supercharged Alfa Romeo Grand Prix car with his un-supercharged Ferrari which, with three times the Alfa's engine displacement, was only its equal in power and certainly its superior in fuel consumption. Every one of the factors governing the power of a piston engine was limited by mechanical considerations, other than the mean effective pressure on the pistons. This is capable of being increased by supercharging, but the increase will not be proportional to the degree of supercharge.
Increasing the mean effective pressure is the same as increasing the volumetric efficiency of the engine: it is a matter of how well the engine breathes and burns its charge of fuel and air. Super-tuning a naturally aspirated engine can improve its volumetric efficiency at certain speeds, at the expense of efficiency at others; and it is broadly limited by the fact that the pressure difference across the inlet valves when they open cannot exceed one atmosphere, that being the ambient pressure in the surrounding air from which the engine breathes.
The most famous era of the blown cars, to our mind, was the 1930s when the Bentleys battled it out with the NAZI backed Mercedes and Auto Union teams. It was the Mercedes team that seemed to win the supercharger reliability stakes, but the interesting thing about their use of the supercharger was that it was not permanently engaged, but rather brought into use by a mechanical linkage when the driver fully depressed the accelerator. Bentley was to quickly learn that this use of the supercharger "only when needed" gave the car a huge reliability advantage - that naturally translated into race success!
Natural Aspiration vs. Supercharging vs. Turbo-Charging
The key difference between a turbocharger and a supercharger is its power supply. Something has to supply the power to run the air compressor. In a supercharger, there is a belt that connects directly to the engine. It gets its power the same way that the water pump or alternator does. A turbocharger, on the other hand, gets its power from the exhaust stream. The exhaust
runs through a turbine, which in turn spins the compressor.
The use of a supercharger creates an artificially dense high-pressure atmosphere from which the engine can ingest a greater rate of air in each operating cycle. But a supercharger, when under load, is thermally inefficient, so its fuel consumption will be higher than that of the highly tuned unblown version of similar power. But the unblown engine may may need a larger number of relatively closely spaced ratios, which introduce problems of their own. The same capacity turbo-charged engine will be mechanically more efficient than the supercharged engine because its turbine does not place a significant overhead on the engine's power output.
The engine with the turbo will likely have more acceptable fuel consumption, but the penalty will be in throttle lag, unless the engine is already running at high speed. If it has been running against a closed throttle for a second or two there will be a delay measured in seconds before the engine will be giving as much torque as it is capable of developing at the speed in question. For that reason race-constructors in the 1960s preferred to use centrifugal superchargers, such as the Miller and the BRM
, although history shows that both were plagued by an abysmal lack of power at low rpm, despite their phenomenal outputs at the other end of the speed range.
The Greenest Engine
Exhaust toxicity is usually measured as concentrations of carbon monoxide, oxides of nitrogen and unburned hydrocarbons in the gases: as a general rule, a highly-tuned unblown engine will be the worst offender with oxides of nitrogen, the supercharged engine worst in unburned hydrocarbons and the turbocharged engine generally the best all-rounder. The turbocharged engine is also likely to be the quietest, the expansion of the exhaust
gases through the turbine quietening the exhaust
at the same time as it extracts power from it that may otherwise go to waste. It is also true that the turbocharged engine is, in many ways, the most efficient - but that is not quite the whole story.
There are two kinds of efficiencies which should be weighed against one another. One is volumetric efficiency, something in which the pressure-charged engine is clearly superior to the normally-aspirated one: the latter seldom (sometimes never) attains 100% volumetric efficiency, whereas the blown engine achieves it at zero boost. The other kind is thermal efficiency, in which the pressure-charged engine, especially if the compressor be driven by the engine rather than by its exhaust
, is equally invariably inferior to the normally-aspirated one.
Thermal efficiency has always been the weakness of a supercharged engine, and no internal-combustion engine
is very good in this respect. Of all the heat units represented by the calorific content of the fuel ingested by the engine, only one third are coverted into useful work: of the remainder, around half, is voided to the atmosphere through the exhaust
system, and the other half is absorbed by the engine, raising the temperature of its components which then shed the heat as best they can, until it, too, is eventually dissipated by the processes of radiation and transfer to the surrounding and cooling air.
The most famous of the early superchargers was the Villiers type, as fitted to the Blower Bentley.
When an engine is supercharged, a greater quantity of heat units flow into it, in direct proportion to the increase in the system's volumetric efficiency. Component temperatures reach much higher levels in a blown engine than in an unblown one. Pistons, piston rings, valves, valve seats and spark plugs are particularly affected, and any localised hot spot in the combustion chamber can provoke pre-ignition of the engine, and consequent destructive detonation. The problem of detonation must also be approached from another angle, intimately related to thermal efficiency of the internal-combustion engine
is simply a heat engine, one in which heat is converted into work and vice versa.
Power and thermal efficiency increase with the compression ratio, but to the thermo-dynamicist it is the expansion ratio that matters, when the heated gases are cooled, the heat loss being converted into useful work. In an unblown engine, the expansion ratio is more or less equivalent to the compression ratio. However the volumetric efficiency of the supercharged engine may be considerably higher than 100% so that the actual compression of the intake charge is greater than the geometrical compression ratio, determined by the cylinder and head dimensions. Thermodynamically the expansion ratio remains unchanged, so there is no possibility of recovery (during the expansion phase) of all the energy potential created during the intensified compression phase.
Turbo's tend to allow wasted heat to escape down the exhaust
pipe when the exhaust
valves are open. One solution is to increase compression and expansion ratios as much as possible, but this risks destructive detonation. In a normally aspirated engine tuned for maximum output, the compression ratio is adjusted so that the combustion chamber pressure at the instant of ignition is as high as possible without running the risk of detonation occurring. Just what that pressure is depends on a number of design features, including the shape of the combusion space, the materials by which it is bounded and the presence of hot spots; but the most critical factor is the fuel itself, which may vary considerably in its anti-knock value or octane rating.
When an engine is supercharged, most of these considerations remain unaltered, as does the maximum pressure that can safely be allowed to build up in the combustion chamber. The mean effective pressure of a supercharged engine may be substantially higher than that of an unsupercharged engine, but the peak pressure can be no higher. The difference in mean pressure must therefore be due to the fact that the rate of pressure drop during the expansion phase is lower, as is confirmed by the fact that there remains scope for further expansion of the combustion gases after the exhaust
valve has opened.
This explains one of the less-appreciated virtues of the supercharged engine, the fact that the mechanical loading on its components does not increase at the same rate as the power: inertia loadings and peak combustion pressures are not significantly different, and in fact the stress reversals in a blown engine may be less severe, because of the greater sustained pressures on the piston during the induction and expansion strokes. In fact the only reason why inertia loads might be higher in a supercharged engine is that it might be necessary to employ a more substantial piston in order to improve the flow of heat through it.
Coming back to the problem of limiting the compression pressure to avoid detonation, if the limit is reached in an unblown engine by raising the geometrical compression ratio as high as is safe, then in a supercharged engine of much higher volumetric efficiency it will be necessary for this geometrical compression ratio to be reduced. The extent to which this is necessary is governed by a third type of efficiency that is crucial to the business of supercharging - adiabatic efficiency. An adiabatic operation is one in which no gain or loss of heat to or from external sources occurs.
When a volume of gas is compressed and cannot escape, the gas pressure rises in proportion to the degree of compression; but the temperature of the gas rises too. It does not rise in strict proportion, the reason for the difference being that whereas the gas is physically trapped, heat is not. In practice, the result is that the charge finally compressed in the cylinder reaches an appreciably higher temperature than it should in a truly adiabatic process. The measure of efficiency of a compressor is how nearly it comes to the true adiabatic state: one of 50% adiabatic efficiency will deliver compressed gas at a temperature twice that of a true adiabatic compression, the extra heat corresponding to losses due to the low pumping efficiency of the compressor.
At any given pressure, the density of a gas in inversely proportional to its temperature, and so the rate of charge is reduced by the heating to which it is subjected. When the charge finally reaches the time for ignition in the combustion chamber, it is the weight of the charge that will determine how much energy can be extracted there and converted into useful work- but it is the temperature of the charge that determines the onset of detonation. For example, a compressor of 50% adiabatic efficiency takes in air at an ambient temperature of 20°C, and delivers it at a pressure of 1.5 atmospheres, the delivery temperature will be 90°C - whereas if the compressor were 75% efficient its outlet temperature would be only 67.3°C.
One way of increasing the adiabatic efficiency of the total compressor system is to employ two-stage supercharging. This can be done with blowers having no internal compression (notably the Roots and the centrifugal types) which simply displace air and deliver it against the resistance of the air already in the inlet manifold. It follows that the work done or power absorbed by the blower is a product of the volume it displaces in a given time and the contra-pressure against which it operates. If an intermediate blower of smaller displacement be inserted in the system, then each of the compressors will be working against significantly smaller contra-pressures, and the total power they absorb will be reduced-typically by more than 20%, which in the case of a Roots blower would be equivalent to raising the pumping efficiency of the system to a level competitive with a single-stage centrifugal blower.
The Roots and Centrifugal Blowers
Although in adiabatic efficiency the Roots and centrifugal blowers belong at opposite ends of the range of compressors used for supercharging, they both belong to a category in which there is no internal compression. All the others do, but most of them have proved after earnest experiment to be unsatisfactory. They include a number of pumps incorporating eccentric vanes, such as the Wittig, the Centric, the Zoller, the Cozette and the Shorrock, of which only the Shorrock lasted into the 1970s. All of them suffer mechanical limitations that prevent them being run at high speeds, and therefore they have to be large and bulky to compensate for this disability. Furthermore, they all demand lubrication, with the risk of oil being entrained with the charge and introduced to the combustion chamber, where it would have a dangerously degrading effect on the resistance of the fuel to detonation.
The same objection applies to the Wankel, which was used by NSU as a supercharger before it was developed as an engine in its own right. These vane blowers have a somewhat higher adiabatic efficiency than the Roots type, most of them being nearly 60% efficiency, and are more suitable for high boost pressures than a single-stage Roots blower which is not very effective above about 1.6 atmospheres. The Roots blower works like a gear-type oil pump, and can be driven at very high speeds. Some leakage from the delivery to the suction side is bound to occur, and this causes pumping losses that grow less significant as the speed of rotation is increased. The delivery characteristics are therefore such that the boost pressure increases with the engine speed up to a certain level - levelling off until it is substantially constant in the topmost portion of the working-speed range.
Different characteristics are displayed by the centrifugal compressor, which has been almost universal in the supercharging of piston aero-engines, has been employed in engine-driven form for sports and racing cars made by Miller, Duesenberg, Auburn and BRM
, and figures in all automotive turbochargers, aviation versions are very refined and commensurately expensive, but even the simpler types used in cars are notably efficient compared with their rivals, commonly exceeding 70% adiabatic efficiency, and occasionally approaching 80%. Mechanically, it is very simple, consisting of a fan or impeller driven at a very high rotational speed, inside a compact casing. Air enters axially and leaves radially, travelling with high velocity imparted by the vanes projecting from the face of the impeller, and passes into a diffuser where its velocity is reduced and the kinetic energy of the air is converted into pressure energy.
The centrifugal compressor is an extremely speed-sensitive device, the output pressure increasing as the-square of the rate of rotation. Thus, a supercharger delivering 5 atmospheres boost at 50,000 rpm will give only 1.25 atmospheres at 25,000. The effect of this upon engine output, and upon the sheer manageability of the car's performance, can be critical to the whole operation, as it was in the case of the V16 BRM in the early 1950s. In that car, raising the engine speed from 7000 to 9000 rpm increased the power output by no less than 80%. The two-stage centrifugal supercharger designed and made by Rolls-Royce for the BRM
gave it extraordinary power, amounting in the final stages of its development to no less than 585 bhp from 1.5 litres, this limit being imposed by available carburettors. In fact, the blowers delivered air at 5.7 atmospheres and at a rate in excess of 1 lb per second, a performance that could not be approached by any other available compressor even had space been available-yet this one measured less than five inches in length and twelve in diameter.
Only its delivery characteristics were criticised, and had the method proposed by Rolls-Royce for controlling the supercharger been adopted (a vortex throttling system which resulted in a constant boost pressure over a satisfactorily wide working range of engine speeds) all this criticism could have been allayed. There remains one shortcoming of the centrifugal supercharger that persists in turbo-charging applications. This is surge - a violent and often audible air disturbance that can amount to anything from a faint pulsation to a violent juddering that can destroy not only the supercharger but also the engine. The surge occurs when the air intake to the compressor is throttled down below some critical point that varies according to the mass flow and pressure ratio. Any attempt to reduce the flow further results in an instantaneous flow reversal, the air travelling back and forth inside the supercharger and growing so hot that the impeller may melt.
A very careful selection of impellers and housings, as well as of turbines, is necessary to match them to an engine, but the number of successful applications of the turbine-driven centrifugal supercharger to racing and road-going cars has increased rapidly over the last decade. Part of the reason is emission regulations, which make it difficult to produce an economical high-performance engine by traditional means, and the turbocharger is an effective alternative.