Supercharging and turbocharging

How mechanical superchargers and exhaust turbochargers raise charge density, with compressor and turbine isentropic relations, intercooling, boost control and turbo lag.

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Why it matters

An engine can burn only as much fuel as the air it traps, so its power is set by the mass of air it swallows per cycle. Supercharging and turbocharging push air in at above-atmospheric density, which lets a smaller engine do the work of a larger one. This is how modern downsized petrol engines and almost every diesel truck, bus, tractor and locomotive engine meet power targets and emission norms at the same time, and how aircraft engines recover power lost at altitude.

Key ideas

Supercharging means supplying the intake charge at a density higher than that of the surrounding atmosphere. Any device that does this is a supercharger in the broad sense; in everyday use "supercharger" means a compressor driven mechanically from the crankshaft (by belt, gears or chain), and "turbocharger" means a compressor driven by an exhaust-gas turbine on the same shaft.

Why density, not just pressure. Indicated power ≈ (mass of air per cycle) × (fuel–air ratio) × (calorific value) × (indicated efficiency). Air mass per cycle = ρ × V_s × η_v. Raising pressure raises density, but compression also raises temperature, which lowers density. That is why the compressor efficiency and an intercooler matter.

Mechanical (positive-displacement or centrifugal) superchargers

  • Roots blower (no internal compression), twin-screw and vane types deliver nearly constant boost across the speed range; centrifugal types give boost that rises roughly with the square of speed.
  • Response is immediate because the drive is mechanical, so there is no lag.
  • The drive power is taken from the crankshaft, so net brake power gain = gain in indicated power − compressor drive power − extra friction. Brake specific fuel consumption usually gets worse.

Turbochargers

  • A radial (centrifugal) compressor and a radial-inflow turbine on one shaft spinning at 100,000 rpm or more, supported in floating or ball bearings fed by engine oil.
  • The turbine recovers part of the energy in the hot, pressurised exhaust, which would otherwise be thrown away. The cost is a higher exhaust back-pressure, which raises pumping work slightly.
  • Turbo lag: after a sudden throttle opening the rotor must accelerate before boost builds; the delay depends on rotor inertia and exhaust energy available at low speed. Remedies: smaller and lighter rotors, ceramic or titanium–aluminide wheels, twin-scroll housings (keep exhaust pulses separate), variable-geometry turbines (VGT, movable nozzle vanes), sequential or twin turbos, and electrically assisted compressors.
  • Boost control: a wastegate bypasses exhaust round the turbine once the target boost is reached; a VGT does the same job by changing the nozzle angle. A blow-off (diverter) valve vents compressed air when the throttle snaps shut, protecting the compressor from surge.
  • Compressor map limits: surge (flow too low for the pressure ratio, flow reversal) on the left; choke (sonic flow) on the right; maximum rotor speed at the top.

Intercooling (charge-air cooling) cools the compressed air in an air-to-air or air-to-water heat exchanger. It raises density further, lowers the compression-end temperature, reduces knock tendency in SI engines and lowers NOx in CI engines.

Effects on the engine

  • Peak cylinder pressure and thermal loading rise, so pistons, rings, bearings, head gaskets and cooling must be designed for it.
  • SI engines are knock-limited: boost is often combined with a lower compression ratio, direct injection, retarded timing near full load and an intercooler.
  • CI engines benefit most: higher air density shortens ignition delay, smoothing combustion, and extra air allows more fuel without smoke. Diesels are almost always turbocharged.
  • Altitude compensation: air density falls with height; supercharging restores sea-level power.

Neighbouring topics: volumetric efficiency (valve timing), knock (SI combustion), delay period (CI combustion), performance testing (bmep, bsfc) and NOx formation all connect here.

Formulas

r_p = p₂ / p₁

  • r_p: compressor pressure ratio (–); p₁, p₂: absolute inlet and delivery pressures (Pa). Always use absolute, not gauge, pressures.

p_boost = p₂ − p_atm

  • p_boost: boost (gauge) pressure (Pa); p_atm: atmospheric pressure (Pa).

T₂s = T₁ · r_p^((γ−1)/γ)

  • T₁: compressor inlet temperature (K); T₂s: isentropic delivery temperature (K); γ: ratio of specific heats (1.4 for air). Valid for an ideal gas, reversible adiabatic compression.

η_c = (T₂s − T₁) / (T₂ − T₁)

  • η_c: compressor isentropic efficiency (–), typically 0.65–0.80; T₂: actual delivery temperature (K).

W_c = ṁ_a · c_p · (T₂ − T₁)

  • W_c: compressor power (W); ṁ_a: air mass flow (kg/s); c_p: 1005 J/kg·K for air.

T₄s = T₃ · (p₄ / p₃)^((γ_g−1)/γ_g) and W_t = ṁ_g · c_pg · η_t · (T₃ − T₄s)

  • T₃, p₃: turbine inlet temperature (K) and pressure (Pa); p₄: turbine outlet pressure (Pa); γ_g ≈ 1.33 and c_pg ≈ 1150 J/kg·K for exhaust gas; η_t: turbine isentropic efficiency (–); ṁ_g = ṁ_a + ṁ_f (kg/s).

η_m · W_t = W_c

  • Steady turbocharger power balance; η_m: turbocharger mechanical efficiency (–).

ρ₂ / ρ₁ = (p₂ / p₁) · (T₁ / T₂)

  • Density ratio (ideal gas); this, not the pressure ratio, sets the gain in trapped air mass.

ε = (T₂ − T₃) / (T₂ − T_cool)

  • ε: intercooler effectiveness (–); T₃: air temperature leaving the intercooler (K); T_cool: coolant (ambient air or water) inlet temperature (K).

Worked examples

Example 1 (standard): supercharger delivery temperature, power and density gain. Given: air enters at p₁ = 100 kPa, T₁ = 300 K; r_p = 2.0; η_c = 0.72; ṁ_a = 0.15 kg/s; γ = 1.4; c_p = 1.005 kJ/kg·K. An intercooler of effectiveness 0.70 uses ambient air at 300 K.

  1. Isentropic delivery temperature: T₂s = T₁ · r_p^((γ−1)/γ) = 300 × 2.0^0.2857 = 365.7 K.
  2. Actual delivery temperature: T₂ = T₁ + (T₂s − T₁)/η_c = 300 + 65.7/0.72 = 391.3 K.
  3. Compressor power: W_c = ṁ_a · c_p · (T₂ − T₁) = 0.15 × 1.005 × 91.3 = 13.76 kW.
  4. Density ratio without intercooler: ρ₂/ρ₁ = 2.0 × 300/391.3 = 1.53.
  5. After the intercooler: T₃ = T₂ − ε(T₂ − T_cool) = 391.3 − 0.70 × 91.3 = 327.4 K, so ρ₃/ρ₁ = 2.0 × 300/327.4 = 1.83. Answer: T₂ ≈ 391 K, W_c ≈ 13.8 kW; density rises 1.53 times without and 1.83 times with the intercooler. Doubling the pressure does not double the air mass.

Example 2 (GATE level): minimum turbine expansion ratio. The compressor of Example 1 is now driven by an exhaust turbine. Exhaust: ṁ_g = 0.155 kg/s, T₃ = 900 K, c_pg = 1.15 kJ/kg·K, γ_g = 1.33, η_t = 0.75, η_m = 0.95. Find the minimum turbine pressure ratio p₃/p₄.

  1. Turbine power needed: W_t = W_c / η_m = 13.76/0.95 = 14.48 kW.
  2. Isentropic temperature drop needed: T₃ − T₄s = W_t / (ṁ_g · c_pg · η_t) = 14.48 / (0.155 × 1.15 × 0.75) = 108.3 K, so T₄s = 791.7 K.
  3. p₃/p₄ = (T₃/T₄s)^(γ_g/(γ_g−1)) = (900/791.7)^(1.33/0.33) = 1.137^4.03 = 1.68. Answer: p₃/p₄ ≈ 1.68. A turbine expanding 1.8 : 1 would give 16.3 kW, i.e. 15.5 kW at the compressor, more than needed, and the wastegate would bleed the excess.

Common mistakes

  • Using gauge pressure in r_p or in the isentropic relation; 1 bar boost at sea level is r_p ≈ 2, not 1.
  • Writing the exponent as γ−1/γ instead of (γ−1)/γ.
  • Calling (r_p^((γ−1)/γ) − 1) an "efficiency"; it is only the fractional isentropic temperature rise.
  • Dividing by η_c the wrong way: the actual temperature rise is larger than the isentropic rise.
  • Assuming air mass rises in proportion to boost pressure, ignoring the temperature rise.
  • Forgetting that a mechanical supercharger's drive power comes off the crankshaft, while a turbocharger's comes mainly from exhaust energy (paid for partly by higher back-pressure).
  • Treating turbo lag as a speed effect only; it is a transient (time) effect after throttle opening.

For GATE ME

Expect numericals that chain the isentropic relation with an efficiency: compressor delivery temperature and power, density ratio with and without an intercooler, turbine–compressor power balance, and power gain of a supercharged engine after deducting drive power. Conceptual questions test why diesels suit turbocharging, knock limits in SI engines, wastegate and VGT functions, surge and choke. Practise keeping pressures absolute and temperatures in kelvin.

Quick check

  1. Air at 1 bar, 300 K is compressed isentropically to 2 bar (γ = 1.4). What is T₂s?
  2. Why does an intercooler increase engine power even though it does not raise pressure?
  3. Which device eliminates lag: a Roots blower or a turbocharger?
  4. What does a wastegate do?
  5. Why is supercharging generally easier to apply to CI engines than to SI engines?

Answers: 1. ≈ 365.7 K. 2. It lowers the charge temperature, so density and trapped air mass increase. 3. The Roots blower, being crank-driven. 4. Bypasses exhaust around the turbine to limit boost. 5. CI engines are not knock-limited; higher charge density actually shortens the ignition delay.

Interview questions

All Automotive Engines, Emissions and Alternate Fuels interview questions

Try answering each one aloud before you open it.

  1. 1.What is supercharging in automotive engines?Concept

    Supercharging is a method of forced induction where a supercharger is used to increase the pressure or density of air supplied to an internal combustion engine. This allows more air and fuel to enter the combustion chamber, resulting in increased power output. Superchargers are typically driven by a belt connected to the engine's crankshaft.

  2. 2.Explain the working principle of a turbocharger.Concept

    A turbocharger has a radial-inflow turbine in the exhaust stream and a centrifugal compressor in the intake, mounted on one shaft. Hot, pressurised exhaust expands through the turbine and drives the compressor, which raises the pressure and density of the intake air so more air, and therefore more fuel, is burned per cycle. Because it recovers energy that would otherwise leave in the exhaust, it costs much less crankshaft power than a mechanical supercharger, although it does raise exhaust back-pressure somewhat. Boost is limited by a wastegate or a variable-geometry turbine, and an intercooler usually cools the compressed air.

  3. 3.Why is turbocharging preferred over supercharging in modern engines?Application

    A mechanical supercharger takes its drive power directly from the crankshaft, so part of the extra power it creates is consumed driving it and brake specific fuel consumption usually rises. A turbocharger is driven mainly by exhaust energy that would otherwise be wasted, so it allows engine downsizing with better fuel economy and lower CO₂ per kilometre. It also suits diesels, which need large air excess for smoke-free combustion. Superchargers are still used where instant response matters, sometimes together with a turbo.

  4. 4.What happens if a turbocharger fails during engine operation?Application

    With a failed turbocharger the engine runs roughly as a naturally aspirated engine with a restrictive turbine in the exhaust, so power and torque fall sharply. In a diesel the fuelling calibrated for boosted air becomes too rich, giving black smoke and high exhaust temperature. Worn bearings or seals let engine oil into the intake or exhaust, giving blue smoke and oil consumption, and a disintegrating wheel can send debris into the engine. The usual root causes are oil starvation, contaminated oil and hot shutdown.

  5. 5.How does a supercharger affect engine performance at low RPMs?Application

    A supercharger can improve engine performance at low RPMs because it is mechanically driven and provides immediate boost pressure. Unlike turbochargers, which may experience lag due to the time required to spool up the turbine, superchargers deliver consistent power across the RPM range, enhancing low-end torque and throttle response.

  6. 6.What are the environmental impacts of using turbochargers in engines?Application

    Turbochargers can have positive environmental impacts by improving engine efficiency and reducing fuel consumption, which in turn lowers CO2 emissions. However, they can also increase NOx emissions due to higher combustion temperatures. Modern engines often use technologies like exhaust gas recirculation (EGR) and selective catalytic reduction (SCR) to mitigate these effects.

  7. 7.A turbocharger compresses air from 1 bar to 2.5 bar (absolute). What are the pressure rise and the pressure ratio?Numerical

    The pressure rise, which is the boost above an atmospheric inlet of 1 bar, is 2.5 − 1 = 1.5 bar. The pressure ratio is 2.5/1 = 2.5, and it is this absolute ratio that goes into the isentropic relation T₂s = T₁·r_p^((γ−1)/γ). For air at 300 K that gives T₂s = 300 × 2.5^0.2857 ≈ 390 K before allowing for compressor inefficiency, which is why an intercooler is usually fitted.

  8. 8.Explain the concept of turbo lag and how it affects engine performance.Concept

    Turbo lag is the delay between the time when the throttle is opened and the time when the turbocharger provides the desired boost pressure. This occurs because the turbocharger relies on exhaust gases to spool up the turbine, which takes time. Turbo lag can affect engine performance by causing a delay in power delivery, especially noticeable during rapid acceleration.

  9. 9.What are some methods used to reduce turbo lag in modern engines?Application

    To reduce turbo lag, manufacturers use several techniques such as variable geometry turbochargers (VGT), which adjust the turbine's geometry to optimize airflow at different engine speeds. Twin-scroll turbochargers separate exhaust pulses to improve turbine efficiency. Additionally, electric turbochargers use electric motors to spool up the turbine quickly, reducing lag.

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