GTU 3171923 Internal Combustion Engine Summer 2026 Solved Paper & Solutions

Given: ideal constant volume (Otto) cycle, $p_1 = 97$ kN/m$^2$, $T_1 = 40 + 273 = 313$ K, $r = 7$, heat supplied $Q_s = 1200$ kJ/kg. Take $\gamma = 1.4$ and $c_v = 0.718$ kJ/kg K.

Check: the heat rejected is $Q_r = Q_s - W = 551$ kJ/kg, and $c_v(T_4 - T_1)$ with $T_4 = T_3/r^{0.4} = 1080$ K gives $0.718(1080 - 313) = 551$ kJ/kg.

Fuel pump: it sucks the fuel from the tank and delivers it at a high pressure and in the correct quantity, at the correct time to each injector. In a diesel engine, the high pressure pump raises the pressure to 150 to 2000 bar, and it controls the quantity of fuel by the load.

Injector: it sprays the fuel into the combustion chamber as a fine mist, at the right time and in the right shape. It atomises the fuel for quick mixing with air, and it opens and closes sharply, with no dribble after the injection.

Spray formation: the fuel leaves the nozzle at a very high velocity (more than 200 m/s). The jet is broken into fine drops by the friction with the dense air in the cylinder and by the turbulence in the jet. The spray has three parts:

1. Atomisation: the breaking of the liquid jet into small drops.
2. Penetration: the distance the spray goes into the air. It must be enough to reach the whole air, but not so much that it hits the cylinder wall.
3. Dispersion (spray cone angle): the spreading of the drops, which gives the mixing with air.

Factors affecting the spray:

• Injection pressure: a higher pressure gives finer drops and more penetration.
• Nozzle hole diameter and design: a small hole gives finer drops.
• Fuel properties: a higher viscosity and surface tension give larger drops, and a poor spray.
• Density and temperature of the air in the cylinder: a denser air gives a shorter penetration and a wider spray.
• Air motion (swirl and turbulence) helps in mixing.

The fuel injection system of a CI engine must give the right quantity at the right time at a high pressure. The injection is of two kinds: air injection (old, not used now) and solid (airless) injection. The types of solid injection are:

1. Individual pump system: each cylinder has its own pump and an injector. The pump is worked by a cam on the camshaft, and a high pressure pipe takes the fuel to the injector. The quantity is controlled by turning the plunger. It is used in large engines.

2. Distributor system: one pump (a rotary pump) supplies the fuel to all the cylinders in turn, through a distributor. It is compact and light, and is used in small and medium engines.

3. Common rail system: a high pressure pump keeps the fuel at a constant high pressure (up to 2000 bar) in a common rail. The injectors, which are electronically controlled (solenoid or piezo) by the ECU, open at the right moment. It gives the pressure independent of engine speed, multiple injections, low noise and low emissions. It is used in modern diesel cars.

4. Unit injector system: the pump and the injector are combined in one unit for each cylinder. It has no high pressure pipe, so there are no pressure losses, and it gives a very high injection pressure.

Combustion in an SI engine has three stages (Ricardo):

1. Ignition lag (preparation phase): the time from the spark to the start of a self-propagating flame. The pressure curve separates from the compression line.
2. Flame propagation (main phase): the flame spreads through the mixture to the whole chamber, and the pressure rises quickly to the peak.
3. After burning: the remaining mixture burns during the expansion stroke, near the walls, with a slow pressure fall.

Knocking is controlled by bringing down the temperature, the pressure and the time spent by the end gas before the flame reaches it.

1. Reduce the compression ratio, which lowers the temperature and pressure of the end gas.
2. Retard the spark timing, so that the peak pressure comes later.
3. Use a high octane fuel, or an anti-knock additive.
4. Increase the turbulence and the flame speed, so the flame reaches the end gas faster.
5. Use a compact combustion chamber, with the spark plug at the centre, to shorten the flame path.
6. Lower the inlet temperature and pressure, with good cooling, and put the hot parts (exhaust valve) away from the end gas.
7. A richer or a leaner mixture than the one that gives the highest knock tendency (about 12:1 to 13:1).

Given: four cylinder, four stroke SI engine, total brake power $BP = 20.9$ kW. Bore $D = 75$ mm, stroke $L = 90$ mm, $N = 3000$ rpm.

Morse test: each cylinder is cut off in turn (by short circuiting the spark plug) at constant speed. The fall in power is the indicated power of that cylinder:

For a four stroke engine, the number of power strokes per second for each cylinder is $n = \dfrac{N}{2\times 60} = 25$. With 4 cylinders:

Emission control reduces pollution, but it has sustainability and ethical issues.

• Sustainability: emission control devices need rare materials (platinum, palladium and rhodium in catalysts), and make energy use and cost higher. A lower tail pipe emission should not be offset by a higher life cycle pollution of making and disposal. CO$_2$ also needs to be reduced for the climate.
• Ethics: the manufacturers must not cheat emission tests (defeat devices, as in the 2015 "dieselgate" case). The health of the public, mainly children and the old, must come before profit. The cost of clean technology should not make it unreachable for poor people.

Biofuels are fuels made from renewable biological material: ethanol and biodiesel.

Ethanol (for SI engines): blended with petrol (E10, E20). It has a high octane number, so it reduces knock, and its oxygen content gives cleaner combustion with less CO and HC. Its energy per litre is lower, so the fuel consumption rises.

Biodiesel (for CI engines): made from vegetable oils or animal fat by transesterification. Blends like B20 can be used in diesel engines with little change. It has a high cetane number and good lubricity. It gives less smoke, CO and HC.

Advantages: renewable, lower net CO$_2$, and reduces dependence on imported oil.

Limitations: a lower calorific value, higher NO$_x$ with biodiesel, poor cold flow, and damage to rubber parts. Also, growing the crops competes with the food and land.

So the biofuels are suitable as blends in engines, with small changes.

Major exhaust emissions:

CO$_2$ is also given out by both, and it is a greenhouse gas.

Emission control measures:

A. Engine (in-cylinder) measures:

1. Better air-fuel control with the electronic fuel injection and the ECU, and a lean burn system.
2. Exhaust gas recirculation (EGR): a part of the exhaust is sent back to the intake. It lowers the combustion temperature, and so reduces NO$_x$.
3. Proper injection timing, high injection pressure (common rail), and a better combustion chamber, which cut smoke and HC.
4. Positive crankcase ventilation (PCV) and an evaporative emission control (a carbon canister) for the HC from the crankcase and the tank.

B. Exhaust after-treatment:

1. Three way catalytic converter (SI): it changes CO, HC and NO$_x$ into CO$_2$, H$_2$O and N$_2$. A platinum, palladium and rhodium catalyst is used, with the lambda sensor.
2. Diesel oxidation catalyst (DOC) for CO and HC, and a diesel particulate filter (DPF) to trap soot.
3. Selective catalytic reduction (SCR): urea (AdBlue) is sprayed into the exhaust, and it reduces NO$_x$ to nitrogen.

C. Fuel and other measures: a clean fuel (low sulphur, as in BS-VI), alternative fuels (CNG, biofuels, hydrogen), an on-board diagnostics (OBD) system, and regular maintenance.

The Wankel engine is a rotary engine. It has a triangular rotor that turns in an oval (epitrochoid) shaped housing, and so it has no pistons or crank and no valves (only ports).

The three faces of the rotor and the housing make three chambers. As the rotor turns around its eccentric shaft, each chamber goes through suction, compression, power (expansion) and exhaust in turn. The spark plug fires at the compression. So the engine gives one power stroke for each turn of the output shaft. It is smooth, small and light, but has problems of sealing and fuel consumption.

The air box method is used to measure the air consumption of an engine. The engine takes air from a large air box (drum) through a sharp edged orifice.

Why an air box? The engine takes air in pulses. The large volume of the box (about 500 times the swept volume) damps the pulsations, so the flow through the orifice is steady, and the manometer reading is steady.

Working: the manometer measures the pressure drop $h_w$ (in mm of water) across the orifice. It is converted into the head of air:

The volume of air drawn in:

where $C_d$ is the discharge coefficient of the orifice (about 0.6) and $A_o$ is the area of the orifice. The mass flow is $\dot m_a = \rho_a Q_a$.

Multi point fuel injection (MPFI) is a system in which one injector is fitted at the inlet port of each cylinder. The fuel is sprayed on the back of the inlet valve, and it mixes with the air that enters. An electronic control unit (ECU) controls the quantity and timing of the fuel.

Main parts:

1. Fuel tank, electric fuel pump and filter.
2. Fuel rail and a pressure regulator, which keeps the fuel at a constant pressure (about 3 to 4 bar).
3. Injectors: electrically operated solenoid valves, one for each cylinder.
4. Sensors: air flow (MAF) or manifold pressure (MAP), throttle position, engine speed and crank position, coolant temperature, and oxygen (lambda) sensor in the exhaust.
5. ECU: it takes the signals from the sensors and decides the injector pulse width (the time for which the injector is open), so that the air-fuel ratio is correct.

Working: air passes the throttle into the inlet manifold, and the ECU calculates the fuel from the air quantity and the engine condition. The injectors spray it. The oxygen sensor gives feedback to keep the ratio near 14.7:1 for the catalyst.

Types: simultaneous (all injectors at once), group and sequential (each injector in the firing order, which is the best).

Advantages over a carburettor:

1. Exact air-fuel ratio for every speed and load, so there is better fuel economy and lower emissions.
2. More power and a better torque, as the intake passage has no venturi restriction.
3. Equal mixture to all cylinders, because each has its own injector.
4. Easy cold start, quick throttle response, and no icing or vapour lock.
5. It works at any angle (no float), and needs less maintenance. It works with the catalytic converter.