Internal Combustion Engines

Introduction and Context

An internal combustion engine (ICE) converts the chemical energy of fuel into mechanical work by burning the fuel–air mixture inside a cylinder. Although electric propulsion is expanding rapidly, ICE technology remains fundamental to automotive engineering because it provides high energy density, rapid refuelling, established manufacturing systems, and wide applicability in passenger vehicles, commercial vehicles, agricultural machinery, and power generation.

The two principal automotive engine types are the spark-ignition (SI) engine and the compression-ignition (CI) engine. SI engines normally use gasoline or similar volatile fuels and initiate combustion with an electric spark. CI engines normally use diesel fuel and initiate combustion by injecting fuel into air that has been compressed to a sufficiently high temperature. Both types contain pistons, cylinders, connecting rods, a crankshaft, valves, lubrication systems, and cooling systems, but their combustion processes and control strategies differ substantially.

This chapter connects engine construction with thermodynamic principles and measurable performance. It also explains valve trains, combustion, turbocharging, and the calculation of torque, power, efficiency, and fuel consumption.

Learning Objectives

By the end of this chapter, a student should be able to:

1. Describe the construction and operating sequence of SI and CI engines.
2. Explain four-stroke and two-stroke operation and identify major engine components.
3. Compare valve-train arrangements and valve timing.
4. Relate ideal thermodynamic cycles to real engine operation.
5. Explain combustion, abnormal combustion, turbocharging, and intercooling.
6. Calculate displacement, torque, brake power, efficiency, mean effective pressure, and fuel consumption.
7. Interpret performance data and identify practical limitations and regulatory considerations.

1. Engine Architecture and Operating Principles

1.1 Major components

The cylinder block supports the cylinders and often contains coolant passages. The cylinder head closes the cylinders and houses the combustion chamber, valves, ports, spark plugs, or fuel injectors. A piston moves reciprocally in each cylinder and transfers gas pressure through the connecting rod to the crankshaft. The crankshaft converts reciprocating motion into rotation.

Piston rings seal combustion gases, control lubricating oil, and transfer heat to the cylinder wall. The camshaft operates the valves. Bearings support the crankshaft and connecting rod journals. The oil pump circulates lubricant, while the cooling system transfers heat to coolant and then to the atmosphere through the radiator.

The intake system supplies air, the fuel system meters fuel, and the exhaust system removes combustion products. Modern engines also include electronic control units (ECUs), sensors, catalytic converters, exhaust-gas recirculation (EGR), and, in many engines, turbochargers.

1.2 Four-stroke cycle

A four-stroke engine completes one operating cycle in four piston strokes and two crankshaft revolutions, or 720° of crankshaft rotation.

1. Intake: The intake valve is open and the piston moves from top dead centre (TDC) to bottom dead centre (BDC). SI engines generally draw a premixed air–fuel charge; CI engines draw mainly air.
2. Compression: Both valves are closed and the piston moves from BDC to TDC. SI engines compress the premixed charge. CI engines compress air only and inject fuel near the end of compression.
3. Power or expansion: Combustion raises cylinder pressure. The expanding gases push the piston toward BDC.
4. Exhaust: The exhaust valve opens and the piston moves toward TDC, expelling burned gases.

Because a power stroke occurs only once every two crankshaft revolutions per cylinder, a flywheel or the rotational inertia of the crankshaft smooths speed variation. In a multicylinder engine, firing events are phased to produce more uniform torque.

A two-stroke engine completes its cycle in one crankshaft revolution. Intake and exhaust processes overlap with compression and expansion, producing a power event every revolution. Two-stroke engines can achieve high specific power but often suffer from scavenging losses, higher emissions, and lubrication challenges. They are less common in modern road vehicles.

1.3 Spark-ignition and compression-ignition engines

An SI engine uses a relatively homogeneous mixture and an ignition system. Its compression ratio is limited by knock, the uncontrolled auto-ignition of part of the unburned mixture. The throttle commonly regulates intake air quantity, especially in conventional gasoline engines.

A CI engine uses a high compression ratio and directly injects fuel into hot compressed air. The fuel quantity is controlled mainly by injection duration and pressure. CI engines generally achieve higher efficiency because of their higher compression ratio, lean operation, and reduced throttling losses. However, they require robust construction, high-pressure injection equipment, and careful control of particulate and nitrogen-oxide emissions.

The ranges in the table are representative, not universal design limits. Actual values depend on combustion-chamber design, fuel, boost pressure, emissions requirements, and operating conditions.

2. Valve Trains and Gas Exchange

The valve train controls the opening and closing of intake and exhaust valves. Common arrangements include overhead-valve (OHV), single overhead camshaft (SOHC), and double overhead camshaft (DOHC) systems. OHV engines place the camshaft in the block and use lifters, pushrods, and rocker arms. SOHC engines use one camshaft in the cylinder head, while DOHC engines use separate camshafts for intake and exhaust valves.

A typical valve train includes a cam lobe, follower or tappet, valve spring, valve stem, retainer, and rocker arm where applicable. The cam profile determines valve lift, duration, and acceleration. Valve clearance is necessary in many designs to accommodate thermal expansion; hydraulic lash adjusters can compensate automatically.

Valves do not open exactly at TDC or BDC. The intake valve may open before exhaust TDC and close after intake BDC. The exhaust valve may open before expansion BDC and close after exhaust TDC. This timing improves gas exchange because gases possess inertia. Near the end of the exhaust stroke, both valves may be open; this interval is called valve overlap. Excessive overlap can cause mixture loss or unstable idle, whereas insufficient overlap can restrict high-speed breathing.

Variable valve timing changes valve phase, duration, or lift according to speed and load. Variable valve lift can reduce pumping losses and improve volumetric efficiency. At high speed, the available time for gas exchange is short, so port geometry, valve area, lift, and pressure differences become critical.

3. Combustion and Thermodynamic Cycles

3.1 Combustion fundamentals

Combustion is a rapid oxidation reaction that releases heat. For a hydrocarbon fuel represented as C_xH_y, complete stoichiometric combustion may be represented by:

In air, nitrogen accompanies oxygen and is largely unchanged, although high combustion temperatures can produce nitrogen oxides. A stoichiometric mixture contains the chemically correct amount of oxygen for complete combustion. The equivalence ratio is:

where F/A is the fuel-to-air mass ratio. A mixture is rich when φ > 1 and lean when φ < 1. SI engines often operate near stoichiometric conditions when a three-way catalyst is used. CI engines commonly operate overall lean, although locally rich zones may occur near fuel sprays.

Combustion duration, flame speed, turbulence, temperature, pressure, and mixture distribution influence efficiency and emissions. Heat release should occur near the appropriate crank angle. If it occurs too early, pressure opposes the piston and knock risk increases; if too late, expansion work is lost.

3.2 Ideal Otto and Diesel cycles

The ideal Otto cycle approximates SI-engine combustion as constant-volume heat addition. Its air-standard efficiency is:

where r is the compression ratio and γ = c_p/c_v is the ratio of specific heats. This equation assumes an ideal gas, constant specific heats, instantaneous heat addition, no heat loss, and reversible compression and expansion. Real engines have lower efficiency because these assumptions are not satisfied.

The ideal Diesel cycle approximates CI-engine combustion as constant-pressure heat addition. Its efficiency is:

where ρ is the cut-off ratio, the volume at the end of heat addition divided by the volume at its beginning. At the same compression ratio, the ideal Otto cycle has higher efficiency; however, practical CI engines normally use higher compression ratios and operate with lower pumping losses, so they can achieve higher brake thermal efficiency.

The four basic ideal processes are isentropic compression, heat addition, isentropic expansion, and heat rejection. Real engines involve finite-rate combustion, friction, heat transfer, gas exchange, residual gases, nonuniform mixtures, and variable specific heats.

3.3 Compression ratio and displacement

The compression ratio is:

where V_d is swept volume and V_c is clearance volume. For one cylinder:

where B is bore and S is stroke. Total engine displacement is the single-cylinder swept volume multiplied by the number of cylinders.

3.4 Abnormal combustion

SI knock occurs when unburned end-gas auto-ignites ahead of the normal flame front, creating pressure oscillations and potentially damaging pistons, bearings, and cylinder-head components. High intake temperature, excessive compression ratio, advanced ignition timing, and low-octane fuel increase risk. Octane rating measures resistance to spark-ignition knock; it is not a direct measure of energy content.

Pre-ignition occurs when a hot surface or deposit ignites the mixture before the spark. It can produce severe thermal damage. In CI engines, ignition delay allows fuel to accumulate before combustion. If a large quantity burns rapidly after ignition, the pressure-rise rate may become excessive, producing noise and mechanical stress. Injection timing, injection rate shaping, exhaust-gas recirculation, and combustion-chamber design control this behaviour.

4. Turbocharging and Air Management

A turbocharger uses exhaust-gas energy to drive a turbine connected to a compressor. The compressor raises intake pressure, allowing a greater mass of air to enter the cylinder. Additional fuel can then be burned while maintaining an appropriate mixture, increasing torque and power without proportionally increasing engine displacement.

A turbocharger includes a turbine, compressor, shaft, bearings, housings, wastegate or variable turbine geometry, and lubrication passages. An intercooler reduces the temperature of compressed air. Since cooler air is denser, intercooling increases charge density and reduces the tendency to knock in SI engines.

The approximate compressor pressure ratio is:

where p₁ and p₂ are compressor inlet and outlet absolute pressures. Gauge pressure must not be used directly in this ratio. For an ideal-gas approximation at constant pressure, density varies inversely with absolute temperature:

where ρ is density, p is absolute pressure, R is the specific gas constant, and T is absolute temperature.

Turbocharging increases thermal and mechanical loading. Excessive boost can cause knock, high exhaust temperature, compressor surge, turbine overspeed, or bearing failure. A wastegate bypasses exhaust around the turbine when boost must be limited. Variable-geometry turbochargers alter turbine flow area and improve response over a wider speed range. Turbo-lag results from the time required to accelerate the rotating assembly and build exhaust energy.

Superchargers perform the same broad function but are mechanically driven by the engine. They provide rapid response but consume crankshaft power. Electric boosting and electrically assisted turbochargers reduce response delay and are increasingly integrated with hybrid electrical systems.

5. Engine Performance Parameters

5.1 Torque and brake power

Torque is the turning moment produced at the crankshaft:

where F is tangential force in newtons and r_m is moment arm in metres. Brake power is the useful power measured at the crankshaft:

where P_b is brake power in watts, N is rotational speed in revolutions per minute, and T is torque in newton-metres. The factor 60 converts minutes to seconds.

A convenient engineering form is:

This applies when torque is in N·m and speed is in r/min.

Indicated power is the power developed by gas pressure inside the cylinders. Friction power is the difference between indicated and brake power:

Mechanical efficiency is:

5.2 Mean effective pressure

Mean effective pressure (MEP) is a fictitious constant pressure that would produce the measured work over the displacement volume. Brake MEP is:

where W_b is brake work per cycle and V_d,total is total displacement. For a four-stroke engine:

For a two-stroke engine:

Torque is therefore directly related to MEP and displacement. MEP allows comparison of engines of different sizes.

5.3 Efficiencies and fuel consumption

Brake thermal efficiency is:

where ṁ_f is fuel mass flow rate in kg/s and LHV is lower heating value in J/kg. The lower heating value is normally used because water in the exhaust is treated as vapour and its condensation energy is not recovered.

Brake-specific fuel consumption is:

when fuel flow is in kg/s and brake power is in kW, giving bsfc in kg/kWh. Lower bsfc indicates better fuel economy at the stated operating point, but it does not by itself describe emissions, transient performance, or total vehicle consumption.

Volumetric efficiency is the actual inducted air mass divided by the reference air mass that would occupy the displacement volume at specified inlet conditions. It may exceed 100% in a boosted engine, depending on the reference definition.

6. Worked Engineering Examples

Example 1: Displacement and compression ratio

A four-cylinder engine has a 86 mm bore and 86 mm stroke. Clearance volume per cylinder is 54 cm³. Determine total displacement and compression ratio.

Convert dimensions: B = S = 0.086\ m.

Using SI units, the result is 0.000499\ m³, or approximately 499 cm³ per cylinder. Thus:

The compression ratio is:

The calculation assumes the stated clearance volume is per cylinder and that bore and stroke are uniform.

Example 2: Torque and brake power

An engine produces 220 N·m at 3000 r/min. Find brake power.

Therefore, the engine produces approximately 69.1 kW, equivalent to about 92.7 mechanical horsepower using 1 hp ≈ 0.7457 kW.

Example 3: Brake thermal efficiency and fuel consumption

A diesel engine produces 80 kW while consuming fuel at 18 kg/h. The fuel LHV is 42.5 MJ/kg. Determine brake thermal efficiency and bsfc.

Convert fuel flow:

Fuel energy rate is:

Therefore:

Specific fuel consumption is:

The result is valid for steady operation and assumes the supplied LHV and measured brake power are accurate.

7. Applied Case: Diagnosing Low Power in a Turbocharged Diesel

Background. An illustrative fleet vehicle with a turbocharged four-cylinder diesel engine develops normal idle speed but loses power under load. Fuel consumption has increased and black smoke appears during acceleration.

Problem. The symptoms indicate that fuel may be available while the air supply is insufficient, causing locally rich combustion. Possible causes include a blocked air filter, a split charge-air hose, a sticking wastegate or variable-geometry mechanism, a faulty boost-pressure sensor, injector malfunction, or excessive exhaust restriction.

Analysis. A technician first records engine speed, boost pressure, exhaust temperature, smoke behaviour, and fuel-rail pressure under controlled load. Diagnostic fault codes are read, but a code-free system is not assumed to be healthy. Inspection then proceeds from simple causes to complex causes: air filter and ducting, intercooler leaks, actuator movement, sensor plausibility, exhaust restriction, and injector balance.

Approach and solution. Suppose inspection finds a split intercooler hose. The compressor produces pressure, but a substantial portion of the charge escapes before reaching the intake manifold. The ECU may increase fuel delivery in response to load, while the cylinders receive inadequate air. Replacing the hose, checking clamps, cleaning the pressure sensor, and verifying boost under load restore the intended air mass.

Outcome. After repair, smoke decreases, measured boost follows the manufacturer’s target, power returns, and fuel consumption improves. The repair is confirmed by a repeatable road or dynamometer test rather than by subjective driving impressions alone.

Learning points. Engine diagnosis should connect symptoms to physical principles. Black smoke in a diesel often suggests an air–fuel imbalance, but it is not conclusive by itself. Measurement, safe test procedures, and emissions compliance are essential. Unauthorised increases in boost or fuel quantity can damage the engine and violate regulations.

8. Limitations, Regulation, and Contemporary Practice

Real engine performance varies with altitude, ambient temperature, humidity, fuel quality, oil viscosity, coolant temperature, accessory load, and transient operation. Dynamometer results must state correction procedures and test conditions. Rated power is meaningful only when speed, standard conditions, and measurement method are specified.

Combustion produces carbon dioxide, carbon monoxide, unburned hydrocarbons, nitrogen oxides, and, particularly in diesel engines, particulate matter. Catalytic converters, particulate filters, selective catalytic reduction, EGR, precise injection, and closed-loop control reduce emissions. These systems introduce pressure losses, thermal-management requirements, maintenance needs, and possible failure modes.

Modern ECUs use crankshaft and camshaft position, manifold pressure, airflow, temperature, oxygen, knock, and exhaust sensors to control injection and ignition. Data-driven and AI-assisted tools can identify abnormal patterns in fleet data, predict component failure, and optimise calibration. Such tools should support—not replace—engineering validation, sensor-quality checks, cybersecurity controls, and human safety decisions.

Best practice includes correct lubrication, filtration, cooling-system maintenance, fuel-system cleanliness, manufacturer-approved calibration, safe isolation during service, and responsible disposal of oils, coolants, filters, and batteries. Technicians must not defeat emissions-control systems or use unsafe modifications merely to obtain short-term performance gains.

Chapter Summary

Internal combustion engines convert fuel chemical energy into crankshaft work through controlled combustion. SI engines initiate combustion with a spark and generally use premixed charge, while CI engines inject fuel into hot compressed air. The four-stroke cycle consists of intake, compression, power, and exhaust; valve timing and overlap strongly influence gas exchange and performance.

The ideal Otto and Diesel cycles provide useful reference models, although real engines experience heat loss, friction, finite combustion duration, pumping work, and nonuniform mixtures. Compression ratio, displacement, MEP, torque, speed, and fuel flow provide the basis for quantitative performance analysis. Brake power is calculated from torque and rotational speed, while brake thermal efficiency relates output power to fuel energy input.

Turbochargers use exhaust energy to increase intake-air mass and engine power, but they also increase thermal and mechanical stresses. Intercooling, wastegates, variable geometry, and electronic control manage these effects. Effective professional practice combines theory, measurement, diagnosis, safety, emissions compliance, and responsible technological development.

Key Terms

Spark-ignition engine: An engine in which a spark initiates combustion.

Compression-ignition engine: An engine in which fuel ignites because of hot compressed air.

Top dead centre (TDC): Piston position nearest the cylinder head.

Bottom dead centre (BDC): Piston position farthest from the cylinder head.

Bore: Internal cylinder diameter.

Stroke: Distance travelled by the piston between TDC and BDC.

Displacement: Swept volume of the piston or total engine swept volume.

Compression ratio: Maximum cylinder volume divided by minimum cylinder volume.

Valve overlap: Period during which intake and exhaust valves are simultaneously open.

Volumetric efficiency: Measure of the effectiveness of cylinder charge filling.

Knock: Abnormal auto-ignition producing damaging pressure oscillations.

Pre-ignition: Combustion initiated before the intended spark.

Turbocharger: Exhaust-driven turbine–compressor system that increases intake pressure.

Intercooler: Heat exchanger that cools compressed intake air.

Torque: Rotational moment produced by the engine.

Brake power: Useful power measured at the crankshaft.

Mean effective pressure: Fictitious average pressure representing work per displacement volume.

Brake thermal efficiency: Brake power divided by fuel energy rate.

Brake-specific fuel consumption: Fuel mass consumed per unit brake energy output.

Equivalence ratio: Actual fuel-to-air ratio divided by stoichiometric fuel-to-air ratio.

Review Questions

Short Answer Questions

1. What are the four strokes of a four-stroke engine?
2. Why does a CI engine generally use a higher compression ratio than an SI engine?
3. What is the function of an intercooler?
4. Define brake thermal efficiency.
5. What is valve overlap, and why is it used?

Long Answer Questions

1. Compare the construction, mixture formation, ignition, load control, and emissions characteristics of SI and CI engines.
2. Explain the operation of an OHV, SOHC, and DOHC valve train and discuss the effects of valve timing.
3. Describe the ideal Otto and Diesel cycles and explain why their efficiencies differ from those of real engines.
4. Explain turbocharger operation, including the functions of the turbine, compressor, wastegate, and intercooler.
5. Discuss the principal sources of engine inefficiency and the technologies used to reduce fuel consumption and emissions.

Analytical / Application Questions

1. An engine produces 300 N·m at 2500 r/min. Calculate its brake power in kW and explain what additional information is needed to calculate brake thermal efficiency.
2. A vehicle shows black smoke, reduced boost pressure, and increased fuel consumption under load. Develop a logical diagnostic sequence and justify each step using engine principles.
3. Two engines have equal displacement, but one produces greater torque at the same speed. Use MEP, air handling, combustion, and friction concepts to explain possible reasons.

Multiple Choice Questions with Answers

Question 1

Which event initiates combustion in a conventional spark-ignition engine?

A. Fuel injection into hot compressed air

B. An electric spark

C. Exhaust-valve opening

D. Turbocharger operation

Correct Answer: B. An electric spark

Question 2

A four-stroke engine completes one thermodynamic cycle in how many crankshaft revolutions?

A. One-half

B. One

C. Two

D. Four

Correct Answer: C. Two

Question 3

An engine produces 200 N·m at 3000 r/min. What is its approximate brake power?

A. 31.4 kW

B. 62.8 kW

C. 94.2 kW

D. 120.0 kW

Correct Answer: B. 62.8 kW

Question 4

Which component converts exhaust-gas energy into shaft rotation in a turbocharger?

A. Compressor

B. Intercooler

C. Turbine

D. Throttle plate

Correct Answer: C. Turbine

Question 5

For a mixture with equivalence ratio φ less than 1, the mixture is described as:

A. Rich

B. Stoichiometric

C. Lean

D. Noncombustible by definition

Correct Answer: C. Lean

Question 6

Which equation represents brake thermal efficiency?

A. Fuel flow divided by displacement

B. Brake power divided by fuel energy rate

C. Indicated power divided by torque

D. Torque divided by engine speed

Correct Answer: B. Brake power divided by fuel energy rate

Question 7

What is the principal purpose of an intercooler?

A. To heat exhaust gases

B. To cool compressed intake air

C. To lubricate the turbine

D. To increase fuel temperature only

Correct Answer: B. To cool compressed intake air

Question 8

A major abnormal-combustion concern in an SI engine is:

A. Knock

B. Valve overlap

C. Oil filtration

D. Crankcase ventilation

Correct Answer: A. Knock

Question 9

For a four-stroke engine, brake MEP is related to torque by which expression?

A. bmep = 2πT ÷ Vd,total

B. bmep = 4πT ÷ Vd,total

C. bmep = Vd,total ÷ 4πT

D. bmep = T ÷ N

Correct Answer: B. bmep = 4πT ÷ Vd,total

Question 10

If an engine consumes 20 kg/h of fuel while producing 100 kW, its approximate bsfc is:

A. 0.05 kg/kWh

B. 0.20 kg/kWh

C. 2.0 kg/kWh

D. 20 kg/kWh

Correct Answer: B. 0.20 kg/kWh