Automotive Engine Designs and Diagnosis

Engine Classifications

Engines can be classified based on several factors:

  • Operational cycles

  • Number of cylinders

  • Cylinder arrangement

  • Valve train type

  • Ignition type

  • Cooling systems

  • Fuel type

Operational Cycle Classification

  • Two-Stroke Gasoline: Completes its cycle in one crankshaft revolution. Requires forced induction due to the absence of a complete intake stroke.

  • Four-Stroke Gasoline: Completes its cycle in two crankshaft revolutions (four strokes).

  • Four-Stroke Diesel: Similar to the four-stroke gasoline engine, requiring two crankshaft revolutions.

  • Two-Stroke Diesel: Completes its cycle in one crankshaft revolution. Employs a supercharger to force intake air through cylinder ports.

  • Rotary (Wankel): Utilizes a rotary design to convert pressure into rotating motion, differing from traditional reciprocating pistons.

Four-Stroke Cycle (Gasoline)

  1. Intake Stroke: Piston moves down, intake valve opens, drawing air and fuel into the cylinder due to atmospheric pressure.

  2. Compression Stroke: Piston moves up, both valves closed, compressing the air/fuel mixture, concentrating fuel particles, and raising temperature for ignition.

  3. Power Stroke: Both valves remain closed; the mixture is ignited, expansion forces the piston down, generating power to rotate the crankshaft.

  4. Exhaust Stroke: Piston moves up, exhaust valve opens, expelling spent gases.

Classification by Number of Cylinders

Engines come in various cylinder configurations:

  • Current designs include 3, 4, 5, 6, 8, 10, and 12-cylinder engines.

  • 4 and 6-cylinder engines are common in automobiles.

  • 6 and 8-cylinder engines are often used in pickups and light trucks.

Cylinder Arrangement Classification

Cylinders can be arranged in different configurations within the cylinder block:

  • In-line Engine: Cylinders are placed in a row (simplest design).

  • Slant Engine: A variation of the in-line, with cylinders placed at an angle for lower hood lines.

  • V-Type Engine: Offers lower hood lines and shorter block due to two rows of cylinders, typically at a 60° or 90° angle.

  • Opposed Engine: Two banks of cylinders face opposite directions from a central crankshaft, often used in rear-engine vehicles with limited space.

Valve Train Classification

  • Overhead Valve (OHV): Valves are in the cylinder head, operated by a camshaft in the block using valve lifters, push rods, and rocker arms.

  • Overhead Camshaft (OHC) / Dual Overhead Camshaft (DOHC): Valves are in the cylinder head, operated directly by the camshaft or cam followers. Some engines use separate camshafts for intake and exhaust.

Ignition Type Classification

  • Spark Ignition: Used in gasoline engines, requiring an electrical system and spark plug to ignite the air/fuel mixture.

  • Compression Ignition: Used in diesel engines, fuel injectors deliver fuel to highly compressed air, where the heat of compression ignites the fuel.

Cooling System Classification

  • Liquid Cooled Engines: Coolant circulates through the engine, removing excess heat and transferring it to the outside air via a radiator.

  • Air Cooled Engines: Airflow through fins on the cylinder block and head removes excess heat.

Fuel Type Classification

  • Gasoline: Used in gasoline engines, mixed with air during the intake stroke for ignition.

  • Diesel: Used in diesel engines, injected directly into the cylinder, igniting upon contact with hot, compressed air.

Gasoline Engine Systems

  • Air/Fuel System: Ensures the engine receives the correct air/fuel mixture for efficient operation. Older engines used carburetors, while modern engines use computer-controlled fuel injectors.

  • Ignition System: Delivers a spark to ignite the air/fuel mixture near the end of the compression stroke. The firing order determines the sequence of spark delivery. Timing is crucial.

  • Lubrication System: Supplies oil to moving parts for lubrication, cleaning, and cooling, reducing friction and wear while transferring heat.

  • Cooling System: Coolant circulates through water jackets to maintain optimal operating temperature, regulated by a thermostat.

  • Exhaust System: Removes burned gases and limits engine noise, carrying carbon monoxide away from the passenger compartment.

  • Emission Controls: Reduces pollutants released into the atmosphere, often achieved through engine refinements like reshaped combustion chambers and variable valve timing.

Engine Measurement and Performance

  • Bore: Cylinder diameter, measured in mm or inches.

  • Stroke: Piston travel length between Top Dead Center (TDC) and Bottom Dead Center (BDC). Bore and stroke determine cylinder displacement.

  • Crankshaft Throw: Distance from the crankshaft's main bearing journal center to the connecting rod bearing journal center. Determines the engine's stroke. 2×Throw=Stroke2 \times \text{Throw} = \text{Stroke}

  • Bore to Stroke Ratio: Oversquare (larger bore than stroke) allows for larger valves and higher engine speeds. Undersquare (larger stroke than bore) produces more power at lower speeds. Square engines have equal bore and stroke measurements.

  • Displacement: Swept volume of a cylinder. Engine displacement is the sum of each cylinder's displacement. Larger displacement often correlates with higher torque.

  • Compression Ratio: The ratio of cylinder volume at BDC to volume at TDC, indicating the compression of the air/fuel mixture.

Engine Efficiency

  • Engine Efficiency: The ratio of energy input to energy output.

  • Volumetric Efficiency: The engine's ability to fill cylinders with the air/fuel mixture during intake. It compares actual intake volume to total cylinder volume. Turbo- or supercharged engines can exceed 100%.

  • Thermal Efficiency: How much heat from combustion is converted into usable power. Typically, only one-third is used for propulsion.

  • Mechanical Efficiency: The ratio of power output to power exerted on pistons, accounting for power losses due to friction. Minimizing friction increases mechanical efficiency.

Torque and Horsepower

  • Torque: A twisting or turning force.

  • Horsepower: The rate at which torque is produced. Engine torque varies with crankshaft speed and other factors.

Basic Engine Tests

  • Compression Test: Assesses cylinder compression using a gauge in kPa and psi. Most gauges have a vent valve to hold the highest reading.

  • Cylinder Leakage Test: Measures compression loss percentage. Compressed air is applied to the cylinder, and leakage location is identified by listening around engine parts.

  • Power Balance Test: Verifies equal power production across cylinders. Spark plugs are shorted one at a time, and engine speed change is recorded. Equal power output results in uniform speed drops.

  • Vacuum Test: Diagnoses engine condition by measuring intake manifold vacuum with a gauge. Vacuum results from piston movement during the intake stroke; maximum vacuum indicates a sealed cylinder.

Vacuum gauge readings can help diagnose several engine conditions:

  • Late Ignition Timing

  • Manifold Leak

  • Weak Valve Spring

  • Leaking Head Gasket

  • Carburetor or Injector Adjustment

  • Burnt or Leaking Valves

  • Sticking Valves

  • Restricted Catalytic Converter or Muffler

Common Engine Noises

  • Ring Noise: High-pitched rattling/clicking during acceleration, caused by worn rings/cylinders, broken piston ring lands, or insufficient ring tension. Repair involves replacing rings, pistons, or reboring cylinders.

  • Piston Slap: Hollow, bell-like sound when the engine is cold, often louder during acceleration. Caused by worn pistons/cylinders, collapsed piston skirts, misaligned connecting rods, or excessive piston-to-cylinder wall clearance.

  • Piston Pin Knock: Sharp, metallic rap, possibly a rattle if all pins are loose, most noticeable at idle when hot. Caused by worn piston pin, piston pin boss/bushing, or insufficient lubrication.

  • Ridge Noise: High-pitched rapping/clicking during deceleration, caused by the piston ring striking the ridge at the top of the cylinder when new rings are installed without removing the old ridge, or loose piston pin / connecting rod bearing.

  • Rod-Bearing Noise: Varies from light tap to heavy knock, caused by worn or loose connecting rod bearings. The noise can lessen by shorting the affected cylinder's spark plug.

  • Main or Thrust Bearing Noise: Loose crankshaft main bearing produces a dull, steady knock. A loose crankshaft thrust bearing results in a heavy thump at irregular intervals, particularly during hard acceleration.

  • Tappet Noise: Light, regular clicking, more noticeable at idle, caused by excessive valve train clearance. Located with a feeler gauge. Can be caused by improper valve adjustment, worn parts, dirty hydraulic lifters, or lack of lubrication.

  • Ping or Detonation: Knocking noise, most noticeable when accelerating a loaded engine at operating temperature. Caused by advanced ignition timing, carbon buildup, low octane fuel or carbon deposits that pre-ignite the air/fuel mixture.