The diesel engine is a compression-ignition engine (self-ignition). Unlike the gasoline engine (spark-ignited via spark plug), air is compressed at a high volumetric ratio (14:1 to 25:1), which raises its temperature beyond the fuel's self-ignition point. The fuel is then injected directly into the combustion chamber under very high pressure (200 to 2,500 bars depending on the generation of the system).
Theoretical diesel cycle (modified Beau de Rochas cycle): adiabatic compression, constant-pressure combustion, adiabatic expansion, constant-volume exhaust. In practice, modern engines operate on a mixed cycle (partially constant-volume combustion, partially constant-pressure combustion).
Fundamental Differences from the Gasoline Engine
Parameter
Diesel Engine
Gasoline Engine
Ignition
Compression (self-ignition)
Spark plug
Compression ratio
14:1 to 25:1
8:1 to 12:1
Intake
Air only
Air/fuel mixture
Power regulation
Quantity of fuel injected
Throttle plate (quantity of air)
Thermal efficiency
35 to 45%
25 to 30%
Torque
High at low RPM
Maximum at medium/high RPM
Diesel Engine Classification
By cycle: 2-stroke (rare in trucks, used in marine/generator sets) or 4-stroke (standard in highway transport).
By injection: direct injection (DI) — chamber in the piston, or indirect injection (IDI) — pre-chamber or turbulence chamber (obsolete technology, only found on older engines).
By aspiration: naturally aspirated, turbocharged, turbocharged with charge air cooler (aftercooler).
Diesel Fuel
Essential Physico-Chemical Properties
Diesel fuel is a mixture of hydrocarbons (C₁₀ to C₂₂) derived from crude oil refining. Its critical properties for the technician:
Cetane number: a measure of the ease of self-ignition (analogous to octane number for gasoline, but inverted). Canadian standard: minimum 40 (generally 42 to 50 for transportation). A number that is too low causes a long ignition delay, diesel knock, and cold-start difficulties.
Pour point: the minimum temperature at which the fuel will still flow. In Canada, winter fuels are formulated for pour points of -40 °C to -48 °C depending on the region.
Cloud point: the temperature at which paraffin crystals begin to form. The fuel filter plugs at this temperature.
Viscosity: affects injection pump lubrication and atomization. Too low = wear of precision parts; too high = poor spray pattern.
Sulfur content: Ultra Low Sulfur Diesel (ULSD, ≤ 15 ppm) has been mandatory in Canada since 2006 for on-road vehicles (Sulphur in Diesel Fuel Regulations, Environment and Climate Change Canada).
Applicable Canadian Standards
CAN/CGSB-3.517: specification for diesel fuel for compression-ignition engines (transportation-grade diesel).
CSA B149.1: Natural Gas and Propane Code — applicable only if the vehicle is converted to compressed natural gas (CNG) or propane. For diesel systems, this standard applies to bi-fuel engines (diesel + gas). Rule 6.24: requirements for CNG cylinders mounted on vehicles.
Additives and Contaminants
Additives: cetane improvers, detergents, anti-foaming agents, lubricity additives (compensating for low sulfur content), anti-gel (pour point depressants), biocides (to prevent microbial growth in tanks).
Common contaminants: water (causes corrosion, rust, bacterial growth, cavitation in injectors), particulates (abrasive wear of injectors and pumps), microbes (black sludge that plugs filters).
Water check procedure: drain the water separator on the fuel filter at the manufacturer's interval (often indicated by a warning light on newer vehicles). Use a water-finding paste (e.g., Kolor Kut) to detect the presence of water in a sample.
Diesel Injection Systems
Conventional Mechanical Injection
In-line pump: each cylinder has its own pump element (plunger). Flow is controlled by a rack that rotates the plunger to modify the effective stroke. Used on older engines (Cummins N14, early Detroit Diesel Series 60).
Rotary distributor pump: a single rotating piston distributes fuel to each injector via a rotary distributor. Examples: Bosch VE, Stanadyne DB4. Flow control via solenoid valve or mechanical governor.
Mechanical injectors: open when fuel pressure exceeds the spring's calibration pressure (generally 150 to 300 bars). The opening pressure is verified on an injector test bench. The spray pattern must have the correct spray angle and fine atomization (mist).
Electronic Injection (HEUI, EUI, Common Rail)
EUI (Electronic Unit Injector): integrated pump-injector, actuated by the camshaft lobe. Electronics (ECM) control a regulating solenoid valve. Injection pressure up to 2,000 bars. Used on Detroit Diesel Series 60, Cummins N14 (electronic versions), Volvo.
HEUI (Hydraulically actuated, Electronically controlled Unit Injector): the injector is actuated hydraulically by high-pressure engine oil (up to 330 bars). The ECM controls a solenoid valve that directs oil to an intensifier piston. Injection pressure up to 1,500 bars. Used on Navistar (International) 7.3 L and 6.0 L engines.
Common Rail: a high-pressure pump feeds a common rail (accumulator) that supplies all injectors. Each injector is individually controlled by the ECM via a solenoid valve or piezoelectric actuator. Pressures from 1,600 to 2,500 bars. Advantages: precise control of injection timing, duration, and pressure; multiple injections (pilot injection, main injection, post-injection).
System Comparison
System
Typical Pressure
Control
Advantages
Disadvantages
In-line pump
200–400 bars
Mechanical
Robust, simple
Manual adjustment, high emissions
Rotary distributor pump
300–700 bars
Mechanical/electronic
Compact, economical
Pressure limit
EUI
1,200–2,000 bars
Electronic
High pressure, precision
High cost, cam wear
HEUI
1,000–1,500 bars
Electronic
Independent of cam
Hydraulic complexity
Common Rail
1,600–2,500 bars
Electronic
Flexible, multi-injection
Sensitive to contaminants
Critical Components and Procedures
Fuel filter: replace at the manufacturer's interval (often 25,000 to 40,000 km). Always prime the system after replacement (manual or electric primer pump). Never start a common rail engine without bleeding the air — risk of cavitation and damage to the high-pressure pump.
Injection pump: check the transfer pressure (low-pressure pump) which should typically be 3 to 7 bars depending on the system. On common rail systems, the high-pressure pump (CP3, CP4) generates the rail pressure — verify with the diagnostic tool.
Injectors: leak-off return test — excessive return indicates a worn injector. On common rail systems, measure each injector's return flow into graduated cylinders; an injector that returns more than 30% more than the others is suspect.
Fuel flow calculation: flow rate (L/h) = fuel consumption (L/100 km) × speed (km/h) ÷ 100. Example: a truck consumes 35 L/100 km at 90 km/h → flow rate = 35 × 90 ÷ 100 = 31.5 L/h.
Electronic Engine Management (ECM)
Sensors and Actuators
The ECM (Engine Control Module) receives signals from sensors and commands actuators. Essential sensors:
Crankshaft position sensor (CKP): inductive or Hall-effect type. Essential for injection synchronization. A failure = no start.
Camshaft position sensor (CMP): identifies the cylinder in the compression stroke. Required for sequential injection.
Manifold absolute pressure sensor (MAP): measures engine load. Used to calculate the mass of intake air.
Engine coolant temperature sensor (ECT): correction of injection timing and cold-start enrichment.
Intake air temperature sensor (IAT): correction for air density.
Fuel rail pressure sensor (FRP): on common rail, measures the pressure in the rail. Feedback loop for regulation.
Mass air flow sensor (MAF): measures the actual air mass. Used for fuel flow calculation and DPF regeneration.
Injection timing: injection advance (in degrees of crankshaft rotation before TDC). When cold, the advance is increased to reduce ignition delay. At full load, the advance is optimized for torque and emissions.
Injection duration: determines the quantity of fuel injected. Expressed in microseconds (µs) or degrees of crankshaft rotation.
Pilot injection: a small injection (1 to 3 mm³) 10 to 30° before the main injection. Reduces combustion noise and NOₓ emissions.
Post-injection: injection after the main injection for DPF regeneration (increases exhaust gas temperature).
The turbocharger uses exhaust gas energy to compress intake air. Components: turbine (hot side), compressor (cold side), shaft, bearings (ball or journal), housing.
Operating parameters:
Boost pressure: typically 0.5 to 2.5 bars (7 to 36 psi) depending on the engine. Measured with a gauge or via the MAP sensor.
Rotational speed: up to 150,000 to 200,000 RPM.
Exhaust gas temperature (EGT): up to 700-800 °C at full load. Excessive EGT (> 750 °C sustained) can damage the turbine and valves.
Common failures: excessive radial or axial play (bearing wear), oil leaks (worn seals), wheel/housing contact (breakage), soot buildup (imbalance).
Inspection procedure: engine off, remove the intake duct and check radial play (push the wheel perpendicular to the shaft) and axial play (push along the shaft). Typical radial play: 0.3 to 0.8 mm; axial play: 0.1 to 0.3 mm. Consult manufacturer specifications.
Charge Air Cooler (Aftercooler)
Cools the compressed air (which heats up during compression) before it enters the intake. Denser air = more oxygen = more power and fewer emissions. Typical pressure drop: 1 to 3 psi (0.07 to 0.2 bar). Check for leaks (pressure test at 10-15 psi with a plug and gauge).
Wastegate and Variable Geometry
Wastegate: a valve that bypasses the turbine to limit boost pressure. Controlled by a pneumatic actuator (controlled by the ECM via a solenoid valve) or electronically.
Variable Geometry Turbocharger (VGT): adjustable vanes around the turbine. At low RPM, the vanes close to increase gas velocity; at high RPM, they open to prevent overpressure. Controlled by the ECM via an electric or pneumatic actuator. A fouled VGT causes a lack of power and over/under-boost codes.
Exhaust System and Aftertreatment
EGR (Exhaust Gas Recirculation): recirculates a portion of exhaust gas back into the intake to reduce combustion temperature and therefore NOₓ. EGR cooler, EGR valve (controlled by the ECM). A fouled or stuck EGR valve causes a loss of power and excessive emissions.
DPF (Diesel Particulate Filter): a particulate filter that captures soot. Passive regeneration (high exhaust temperature) or active regeneration (post-injection to raise temperature). The DPF must be replaced or cleaned when the differential pressure exceeds the limit (typically 25 kPa at full load).
SCR (Selective Catalytic Reduction): injection of urea (DEF — Diesel Exhaust Fluid, AUS 32) into the exhaust to reduce NOₓ into nitrogen and water vapor. DEF is a 32.5% urea solution in demineralized water. The SCR system includes: DEF tank, pump, injector, SCR catalyst, NOₓ sensors. The vehicle will not start if the DEF tank is empty (per Canadian emissions regulations).
Lubrication System
Role and Specifications
The lubrication system reduces friction, cools, cleans, protects against corrosion, and provides sealing. The lubricant must be selected according to the API (American Petroleum Institute) classification and SAE viscosity.
API classifications for diesel engines:
API CJ-4: for 2007 and newer engines (compatible with aftertreatment systems).
API CK-4: for 2017 and newer engines, backward compatible with previous engines.
API FA-4: for 2017 and newer engines, low high-temperature viscosity (fuel economy), not backward compatible.
SAE viscosity: 15W-40 (standard), 10W-30 (cold climates), 5W-40 (extreme cold). The first number indicates cold viscosity, the second indicates hot viscosity (at 100 °C).
Circuit Components
Oil pan: typical capacity 15 to 40 L depending on the engine.
Oil pump: gear or rotor type. Typical flow: 50 to 150 L/min at rated speed. Pressure: 30 to 60 psi (2 to 4 bars) when hot, 10 psi minimum at idle.
Oil filter: full-flow or bypass filtration. Bypass valve: opens if the filter is plugged (prevents oil starvation).
Oil cooler: water-cooled or air-cooled. Maintains oil temperature between 90 and 110 °C.
Pressure relief valve: limits maximum circuit pressure.
Procedures and Calculations
Level check: engine off, wait 5 minutes after shutdown (to allow oil to drain back to the pan). The level must be between the MIN and MAX marks on the dipstick.
Oil change interval: per the manufacturer (often 25,000 to 50,000 km for modern engines with oil analysis). Oil analysis (spectrometry) detects metal wear (iron, copper, lead) and contaminants (soot, water, glycol).
Oil consumption calculation: consumption (L/1,000 km) = volume added (L) ÷ distance traveled (km) × 1,000. Consumption greater than 0.5 L/1,000 km is abnormal for a healthy engine.
Cooling System
Role and Specifications
The cooling system maintains engine temperature within the optimal range (85 to 95 °C for most diesel engines). An engine that is too cold: increased wear, excessive fuel consumption, soot formation. An engine that is too hot: detonation, valve damage, cylinder head cracking.
Components: radiator, thermostat, water pump (centrifugal), fan (mechanical, viscous, electric, or hydraulic), expansion tank, hoses, oil cooler, EGR cooler.
Coolant
Type: 50/50 mixture of ethylene glycol (or propylene glycol) and demineralized water. Freeze protection down to -37 °C and boil protection up to 108 °C (with a 15 psi radiator cap).
Standards: ASTM D3306 (ethylene glycol), ASTM D5216 (propylene glycol). OAT (Organic Acid Technology) coolants have a service life of 5 years or 600,000 km; conventional coolants (IAT): 2 years or 100,000 km.
Freeze point check: refractometer (measures refractive index) or hydrometer (density). A 50/50 mixture freezes at -37 °C; a 30/70 mixture freezes at -16 °C.
Procedures
System pressure test: use a pressure tester (15 psi typical). A pressure drop indicates a leak. Check the radiator cap (maintains pressure) — pressure release test.
System flush: open the drain cock on the engine block, drain the coolant, flush with clean water, close, refill with fresh mixture, bleed the air (via the bleed screw or by running the engine with the expansion tank cap open).
Thermostat: operation test — immerse in hot water with a thermometer. The thermostat must begin to open at the start-to-open temperature (e.g., 82 °C) and be fully open at the full-open temperature (e.g., 95 °C).
Starting and Charging System
Starter Motor
The starter is a DC electric motor that drives the flywheel via a pinion gear. Typical power: 3 to 7 kW for truck diesel engines.
Components: motor (field coils, armature, brushes), solenoid (contactor + electromagnet), drive pinion (Bendix or positive engagement), overrunning clutch.
Diagnostic procedure:
125.Check the battery (open-circuit voltage ≥ 12.4 V; under load ≥ 9.6 V during cranking).
126.Check connections (clean and tight terminals, voltage drop < 0.5 V on the ground side, < 0.3 V on the positive side).
127.Voltage at the solenoid during cranking: ≥ 10.5 V.
128.Cranking current: measure with a clamp meter. Excessive current (> 1,000 A) indicates a mechanical problem (seized engine, oil too viscous) or an electrical problem (short circuit in the armature).
Voltage drop: ΔV = I × R. For a 2 m cable with a resistance of 0.001 Ω/m and a current of 800 A: ΔV = 800 × 0.002 = 1.6 V. A drop greater than 0.5 V per connection is excessive.
Alternator
The alternator charges the battery and powers the electrical systems. Typical output: 100 to 200 A at 12 V (or 24 V on some heavy vehicles).
Charging voltage: 13.8 to 14.5 V at 12 V (27.6 to 29 V at 24 V) at 2,000 RPM.
Residual ripple: < 0.5 V peak-to-peak (measured with an oscilloscope or multimeter in AC mode).
Charging current: measure with a clamp meter — should be close to the alternator's rated current after a cold start.
Voltage regulator: maintains constant voltage (temperature compensation: reduces charging voltage at high battery temperature).
Batteries
Trucks use lead-acid batteries (flooded, AGM, or gel) or lithium-ion (increasingly). Nominal voltage: 12 V (often 2 or 3 batteries in parallel for capacity, or in series for 24 V).
Capacity: expressed in amp-hours (Ah) or Cold Cranking Amps (CCA). A diesel truck typically requires 800 to 1,500 total CCA.
Load test: a conductance tester (e.g., Midtronics) measures the internal conductance of the battery. A healthy battery must have a conductance ≥ 70% of its rated value.
Summary
The diesel engine operates by self-ignition: compressed air (14:1 to 25:1) reaches a temperature sufficient to ignite the injected fuel.
Diesel fuel is characterized by its cetane number (≥ 40), pour point (adapted to Canadian climate), and sulfur content (≤ 15 ppm, ULSD).
Injection systems have evolved from mechanical (in-line pump, distributor pump) to electronic (EUI, HEUI, Common Rail) with pressures reaching 2,500 bars.
The ECM controls injection via sensors (CKP, MAP, ECT, FRP) and actuators; diagnostics are performed using DTC codes (SAE J1939) and live data.
Forced induction (turbocharger, VGT) increases power; aftertreatment (EGR, DPF, SCR) reduces emissions — each component has its specific verification procedures.
Lubrication requires oils meeting API classifications (CJ-4, CK-4) with a viscosity suited to the climate (15W-40 standard).
Cooling maintains the engine between 85 and 95 °C; a 50/50 coolant mixture protects down to -37 °C.
Starting and charging depend on healthy batteries (sufficient CCA), clean connections (voltage drop < 0.5 V), and an alternator producing 13.8 to 14.5 V.
Common Pitfalls to Avoid
154.Confusing cetane number and octane number: cetane measures the ease of self-ignition (diesel), octane measures the resistance to self-ignition (gasoline). A diesel engine requires a HIGH cetane number; a gasoline engine requires a HIGH octane number.
155.Neglecting to prime the fuel system after a filter replacement: on a common rail, starting without bleeding can damage the high-pressure pump through cavitation.
156.Confusing pressures: rail pressure (common rail) is 1,600 to 2,500 bars; transfer pressure (low pressure) is 3 to 7 bars. Do not apply the same test values.
157.Forgetting the regulator's temperature compensation: an alternator charging at 14.5 V in winter can overcharge the battery in summer — the regulator must reduce voltage at high temperatures.
158.Using oil that does not meet the API classification: using API CJ-4 oil in an engine requiring CK-4 can damage the aftertreatment system (SCR catalyst poisoning).
159.Not checking turbocharger play before condemning the ECM: excessive play causes boost codes that are not electronic faults.
160.Ignoring DPF differential pressure: a plugged DPF (ΔP > 25 kPa) causes power loss and overheating — replace or clean it before looking for other causes.
161.Confusing standards: the Canadian Electrical Code (Chapter V) applies to fixed electrical installations; for vehicles, it is CSA B149.1 (gas) or CGSB specifications (fuels) that apply.
162.Forgetting to bleed the cooling system: an air pocket causes localized overheating and a false temperature reading — always bleed after draining.
163.Neglecting ground-side voltage drop: a corroded ground cable can prevent starting even with a charged battery — measure voltage drop under load, not at rest.
