Diesel engine principles (motor propulsion)

DOE-HDBK-1018 Vol.1 §1-1

DOE-HDBK-1018 Vol.1 §1-1 — Diesel operating principle and the four-stroke cycle A diesel is a compression-ignition internal-combustion engine: air alone is drawn in and compressed to roughly 300-500 psi, raising its temperature to about 1000 degrees F, well above the auto-ignition point of the fuel. Finely atomized fuel injected near the end of compression ignites spontaneously from this heat of compression, so no spark plug or carburetor is used. Because the diesel compresses only air, it tolerates much higher compression ratios (14:1 to 24:1) than a spark-ignition engine, which is the root of its superior thermal efficiency. In the four-stroke cycle each cylinder completes power delivery over two crankshaft revolutions and four piston strokes. Intake: the piston moves down with the inta…

DOE-HDBK-1018 Vol.1 §1-2

DOE-HDBK-1018 Vol.1 §1-2 — Two-stroke cycle, scavenging, and supercharging A two-stroke diesel completes intake, compression, power, and exhaust in a single crankshaft revolution (two piston strokes), giving one power stroke per revolution instead of every other revolution. Because there is no separate intake or exhaust stroke, fresh air must be forced in to sweep out combustion products — a process called scavenging. A blower (Roots-type) or turbocharger supplies scavenging air at a pressure above exhaust-manifold pressure. Air enters through ports uncovered by the piston skirt near bottom dead center; exhaust leaves either through poppet valves in the head (uniflow scavenging) or through opposing ports. Uniflow scavenging, with intake ports low and exhaust valves in the head, is the mos…

DOE-HDBK-1018 Vol.1 §1-3

DOE-HDBK-1018 Vol.1 §1-3 — Fuel system and injection The diesel fuel system must deliver a metered, precisely timed, finely atomized charge of clean fuel to each cylinder against very high cylinder pressure. From the tank, fuel passes through primary and secondary filters and a water separator, is raised by a transfer pump, and reaches the high-pressure injection equipment. Atomization breaks the fuel into a fine mist so it mixes rapidly with hot compressed air and burns completely; poor atomization causes incomplete combustion, smoke, and carbon deposits. Three common arrangements exist. In a pump-line-nozzle system a jerk-type injection pump (one plunger per cylinder) develops high pressure and sends it through a line to a spring-loaded injector that opens when line pressure lifts the n…

DOE-HDBK-1018 Vol.1 §1-5

DOE-HDBK-1018 Vol.1 §1-5 — Starting systems and starting air A diesel cannot start itself; an external system must crank it fast enough to generate the compression heat needed for ignition. Small engines use an electric starter motor drawing on a storage battery, engaging the flywheel ring gear through a Bendix or solenoid-shift pinion. Larger marine and stationary diesels use compressed-air starting because it delivers very high torque without huge electrical current. Air stored at about 250-350 psi in receivers is admitted through a manually or pilot-operated master starting valve to air-start valves in each cylinder head; a distributor times the air to whichever cylinders are positioned to produce a downward push, turning the engine until fuel injection takes over and combustion sustai…

DOE-HDBK-1018 Vol.1 §1-8

DOE-HDBK-1018 Vol.1 §1-8 — Combustion, timing, and engine operation terminology Good diesel combustion depends on delivering the right amount of clean, well-atomized fuel, at the right time, into sufficiently hot and dense air. Ignition delay is the short interval between the start of injection and the start of pressure rise; if delay is too long, too much fuel accumulates and burns at once, causing rough running and the characteristic diesel knock. Combustion should be timed so the peak pressure occurs just after top dead center, converting the most heat into useful work. Excess air is always supplied because perfect mixing is impossible; too little air produces black smoke (unburned carbon) and overheating, while a fouled turbocharger or dirty air filter reduces the air supply and has t…

NAVEDTRA 14075 §3-1

NAVEDTRA 14075 §3-1 — Diesel maintenance and performance monitoring Reliable diesel operation depends on systematic maintenance and on watching the engine's own indications of health. The core condition indicators are compression pressure and firing (peak combustion) pressure for each cylinder, exhaust temperature per cylinder, jacket-water and lube-oil temperatures and pressures, turbocharger speed, and fuel consumption. Compression pressure that is low on a cylinder points to leakage past worn rings, a burned or poorly seating valve, or a scored liner; a cylinder running hotter or colder on exhaust than its neighbors points to an injector fault, an intake or exhaust problem, or uneven load sharing. Rising fuel consumption for the same power, falling firing pressures, and increasing smok…