Supercharging and turbocharging (1)
by François Dovat
(© François Dovat)
The mean effective pressure (mep or bmep) - and so the torque and power - of a reciprocating internal combustion engine can be increased in vast proportions by supercharging it with compressed air.
However its compression ratio, i.e. the ratio of the volumes contained in the cylinder and combustion chamber with the piston at the bottom dead center (BDC) and at top dead center (TDC) must in that case be reduced so that the compression pressure (Pcomp) and the maximum combustion pressure (Pmax) do not exceed the acceptable limits. On spark ignition engines the limitation of Pcomp is necessary to avoid the phenomenon of auto-ignition and detonation whereas on diesels Pmax is limited by the mechanical strength of the structures of the engine.
The compressed air (or sometimes in the past, the compressed mixture) is provided by a compressor known as a supercharger and of which there are various types. This compressor must be driven by an unspecified source of power. This source may be either the crankshaft (mechanical drive), or an exhaust turbine (turbocharger), or else an independent auxiliary (slave) engine or even a combination of these various means.
Turbochargers are widespread today on diesels of all sizes, from the tiny 0.8 liters 3 cylinders of the Smart to giant marine ones like the 12 cylinders in line Sulzer of 21715 liters high as a building which develops nothing less than 93360 hp at 102 rpm. Thus, the power absorbed by the compressor is not withdrawn anymore from the crankshaft output, but produced by one or several exhaust turbines. Although these turbines are a hindrance to the exhaust gases flow, the backpressure which results from them is more than compensated by the increase of intake pressure (boost pressure) due to the compressor. Moreover, the boost pressure is simply and automatically regulated according to the load.
However, for a road or rail traction engine constantly subject to load and revs changes, the turbocharging is far from being the ideal solution. The inertia of the rotary parts of the turbo-machinery opposes to an instantaneous rise in the boost pressure when the accelerator is suddenly hammered down. The response delay was prohibitory and a lot of R&D had to be done to shorten it, especially by reducing the diameter of the rotors of turbine and compressors. Moreover, the power of an exhaust turbine is not inversely proportional in a linear way to the entropy of the exhaust gas driving it: thus, when the revs of the crankshaft drop below that for which the turbine was optimized, the power of this turbine falls more quickly than the engine speed. Even today, the result is that the intake pressure collapses when the rpm decrease under about half of the nominal revs. Recently, variable geometry turbines of adjustable inlet section or adjustable stator nozzle blades pitch which minimize these disadvantages are available, but a considerable sum of research and development was necessary before managing to beef up turbocharged engines showing an acceptable behavior for road traction.
It is necessary to notice, that contrary to a widespread belief, a turbocharger does not "switch on". It spins already when the engine is idling and the intake pressure builds up continuously and progressively alongside the rise in load and revs, although there's a spiky rise at some point.
Systems including a small high-revving motor-generator coupled directly on the turbocharger shaft are under development and should be marketed soon. Also, several turbochargers can be set in parallel, per example one for each cylinder banks, or in series for a 2-stage turbocharging (diesels), or else in series with a mechanically driven supercharger. Moreover, the shaft of the turbocharger can be mechanically connected to the crankshaft. Scania and Volvo market turbocompound diesels of this kind, which develop respectively 470 and 500 hp. On this subject see our file "Turbocompounding".
The Formula 1 Renault V6 of the Seventies and Eighties was fed by two KKK turbochargers, one for each cylinders bank. The intake air was directed into air/air and air/water side located intercoolers, this system ensuring a greater stability of the charge temperature along the variations of the car speed. Waste gates located on the exhaust before the turbines limited the boost pressure. The turbocharging does not exclude the use of tuned intake manifolds improving the cylinders filling at low revs.
Turbochargers are always of the aerodynamic kind, either of the axial or centrifugal type, but the centrifugal ones are exclusively used in automotive engines, the axial variety having been used on larger engines. Other than on aircraft and marine engines mechanically driven centrifugal superchargers have also been fitted to some few automotive engines, but the result is an extremely peaky power curve – unless a continuously variable drive is used. Common superchargers are of the volumetric kind, either of the Roots or Lysholm type.
This picture shows how a "variable geometry turbocharger" (VGT) works. As a matter of fact, it is the turbine inlet only which is fitted with variable pitch stator blades. This technology has faced tremendous development difficulties because of the high exhaust temperatures (up to 950°C) in which the blades have to operate. On some recent engines, the blades are swiveled by an electric motor with reduction gear, electronically regulated, instead of a pneumatic cylinder actuated by the vacuum from the servo-brake. Another system restricts the inlet area to the turbine by means of a sliding sleeve.
It is possible to turbocharge a V engine by means of a single unit, like on this Scania DSC 14 truck engine. But since a large one is needed, its rotating parts have more inertia than in the case of two smaller turbochargers.
For this 12 liters 6-in line DAF racing truck the two VTG turbochargers working in parallel are quite large anyway in order to feed the 1500 hp (yes, 1500 !). Boost reaches the hard to believe pressure of 5 bars while the firing pressure attains more than 310 bars and the mean effective pressure is over 52 bars. A pneumatic motor can swivel the inlet turbines blades from an end position to the other in 34 ms. The racing truck accelerates from 60 to 160 km/h in no more than 6.5 sec.
Twin turbochargers - one for each cylinder bank - are there on the 1200hp Perkins CV12 diesel powering the British Army's Challenger main battle tank.
WWII aircraft engines were equipped with huge centrifugal compressors, such as this 44.5 liters inverted V12 DB 603. These superchargers were commonly crankshaft driven by the intermediate of 2-speed or even 3-speed step-up gears. But Daimler-Benz used instead a fluid coupling which was more or less filled to vary the boost pressure and to increase the compressor speed along with the elevation. As the mass flow rate of a centrifugal compressor is roughly proportional to the square of its rotational speed, according to what is called a propeller law, it is therefore suitable for aircraft engines. In Germany turbochargers were outfitted as 1st stages (feeding into a 2nd stage crankshaft driven supercharger) on few prototypes only because the high grade steel necessary to manufacture exhaust turbines was scarce.
This 39 liters 2-stroke Detroit Diesel 16V 149 TI is fed by 4 turbochargers in parallel but nevertheless working in series with two Roots scavenge blowers. Under high load, the Roots blowers are by-passed.
One of the 3-lobes rotors of a Roots supercharger is seen on the cutaway of a Detroit Diesel 8V 92 TA. The single turbocharger is located at the top of the Roots unit on this smaller engine. The air-water "aftercooler" is also visible (in blue) inside the Vee. Roots superchargers are of the positive displacement (volumetric) type as briefly said in page 1. Their mass flow rate being quite proportional to their speed, they have been used to a certain extend on automotive engines, either to feed the scavenge air to 2-stroke diesels or to provide a moderate boost pressure in order to increase the output of 4-stroke engines
The Mercedes M 271 of 1.8 liters cranks out up to192 hp at 5800 rpm, thanks to a improved Roots (Eaton) supercharger with helical rotors and to an intercooler.
The maximum adiabatic efficiency of automotive size compressors is as follow:
Roots (Eaton): 50%
Lysholm : 65%
Centrifugal : 77%
It means that a part of the power used to drive the compressor is wasted in heating the outlet compressed air instead of producing more boost. Unfortunately, because of their speed air-flow characteristics, the superior efficiency of centrifugal compressors is completely unusable on automotive engines when driven by a fixed ratio from the crankshaft.
For the 32 V6 and 55 V8 engines, AMG uses a Lysholm-type supercharger produced by IHI in Japan. Driven at 3.3 times crankshaft speed, the twin screw supercharger develops over 1 bar of boost on the V6 and up to 0.8 bar on the 500 hp V8. One of its two cast aluminium rotors is Teflon-coated (the yellow one), and the supercharger drive is through an air-conditioning style electromagnetic clutch. Despite the good efficiency of this design of supercharger, the blower still requires 60 hp to power it when the 3.2 liter V6 is at its 6200 max rpm. Of course, the blower flows a lot of air, up to 1200 kg per hour in fact, but the drawback remains that this power is subtracted from the engine's output which is then subject to an additional load – of 17 % in this case.
In this regard, turbocharging is superior and the only advantages of positive displacement compressors lays in the absence of the so called " turbo lag "together with a better low end torque. But these advantages have diminished with the availability of variable turbine intake geometry. For high boost applications –over 1 bar – the choice of turbocharging is essential.
But let's agree that a volumetric blower is easily and nicely packaged in the V of a car's V engine. The MB S 55 AMG has impressive performances and quite a good efficiency with its declutchable Lysholm compressor. It provides the same performances as the S 600 V12 and has lower fuel consumption.
Other types of superchargers such as the sliding vane one have also been used in automotive applications, but most of them are obsolete nowadays.On the other hand new concepts also appear.
To know more, here comes a link to an outstanding website: