MEP (mean effective pressure) is the average pressure acting on the piston, and increasing it raises the power output of an engine of given size without increasing engine dimensions, i.e. improves the power-to-weight ratio. Power is proportional to MEP and speed, so a higher MEP (today 18 to 25 bar for large engines) produces more power per cylinder. To obtain a higher MEP, a larger mass of air must be admitted to burn a correspondingly larger mass of fuel, and the air must be supplied at a higher pressure. This is the role of the turbocharger compression ratio - the ratio of the compressor delivery pressure to the compressor inlet (atmospheric) pressure. A higher compressor pressure ratio delivers air at higher density, so more air mass is trapped per cycle and more fuel can be burned, raising MEP.

Modern engines therefore demand turbocharger pressure ratios up to about 4.5 to 5.5:1 (with charge air pressures of 4 to 5 bar), whereas older engines used ratios of about 1.5 to 3:1.

The problem with a single high-pressure-ratio compressor is that as the ratio increases, the compressor operates nearer its surge limit, its efficiency normally falls at the top of the operating range, and the air temperature rise (and hence the work needed to compress) increases markedly. It is also difficult for one stage to deliver both high pressure ratio and high efficiency across the whole engine operating speed range. It is therefore necessary to introduce two-stage turbocharging, where the air is compressed in two turbocharger stages (usually one high-pressure and one low-pressure compressor, often with intercooling between the stages and a charge-air cooler after the low-pressure stage). Two-stage compression:

• achieves a higher overall pressure ratio than a single stage at similar or better efficiency;
• lowers the compressed air temperature for a given delivery pressure (approximately isothermal-like compression), giving cooler, denser air to the engine;
• keeps each compressor operating in its most efficient region and reduces surge risk;
• reduces the work per stage and hence the power taken from the exhaust, and improves engine part-load response.

The higher boost allows engines to exceed the former limits on MEP and thus to achieve significantly higher power-to-weight ratios, with two-stage turbocharging being a key enabler for today's highest-MEP engines.

(i) Which area is varied: the effective flow area of the exhaust gas into the turbine nozzle ring (the nozzle vanes at the turbine entry) is varied. It is the throat/entry area available to the exhaust gas as it passes to the turbine blades.

(ii) Why it is varied: with a fixed area turbine, the turbocharger is matched at one design point. At low engine load the exhaust energy is small, the turbine runs slowly and the compressor delivers too little boost, giving poor air/fuel ratio, black smoke and high thermal load; at high load the fixed nozzle may choke and the turbocharger over-speed. Varying the turbine area adjusts the nozzle area to control the turbine inlet pressure and hence the boost, so that a good air/fuel ratio can be obtained over a wide load range, smoke is reduced at low load, the turbocharger over-speed is limited at high load, and the engine can be optimised for better fuel consumption and lower exhaust temperatures.

(iii) How it is varied: by a control mechanism that rotates the nozzle vanes (swirl vanes or a ring of adjustable vanes) in the turbine entry, hydraulically or pneumatically, driven by the electronic control system which usually bases the vane position (and hence area) on engine speed and load. At low load the vanes close up to reduce the area (heavy throttle effect), increasing exhaust velocity and turbine power and boost; at high load they open to increase the area, avoid choking and prevent overspeed. Alternatively a sliding sleeve or adjustable guide ring alters the nozzle area. The setting may be feedback-controlled to maintain a target charge-air pressure or scavenge pressure.