A diesel engine compresses air alone until it is hot enough to ignite fuel sprayed into it, so it has no spark plug and no carburetor and can run at a much higher compression ratio than a petrol engine without knock. At a ratio of 16 the ideal compression heats air from 20 °C to about 616 °C, and the injection pump that meters and sprays the fuel at high pressure is the part a settlement finds hardest to make.
Part of Internal Combustion
The compression-ignition cycle, how its efficiency compares with the spark-ignition cycle, what fuels it burns, and the failure modes of its injection system.
Air Is Compressed Alone And Fuel Is Added Late
In a spark-ignition engine the fuel and air are mixed first, so the compression has to stay cool enough not to light the mixture early. In a diesel only air is in the cylinder during compression, so nothing can light early. Near the top of the stroke the injector sprays in the fuel, and the compressed air, hotter than the fuel’s self-ignition temperature, lights it. The carrier states the principle: there is no spark plug, and compression alone heats the air to 500+ °C.
The result is a different cycle. Heat is added while the piston moves down, over a stretch of the stroke described by the cutoff ratio ρ (the volume after the burn divided by the volume at the top), and not all at once at constant volume. The ideal efficiency, a recalled textbook result, is
η = 1 − (1 ÷ r^(γ − 1)) × (ρ^γ − 1) ÷ (γ × (ρ − 1)).
The engine is controlled by the amount of fuel injected, not by throttling the air, so it breathes freely at light load. That is one reason diesels are efficient. The carrier notes that they are more efficient than petrol engines and common in trucks, tractors and generators.
The Fuel And The Injection System
The fuel can be biodiesel, as the carrier describes: made from vegetable oil or animal fat with methanol and lye, a fuel that runs in a diesel engine without modification. Details are in Biodiesel Production. The carrier also notes that straight vegetable oil works if preheated to 70 °C to reduce its viscosity, with a two-tank arrangement: start on biodiesel or diesel, switch to hot oil once warm.
The injection pump must deliver a small, precisely measured quantity of fuel, at high pressure, at the right instant, once per power stroke, through an injector that atomises it. Its parts are closely fitted, and the clearances are small. The Precision Hierarchy explains why fits this close depend on a long chain of earlier tools and gauges. The practical advice is to salvage pumps and injectors, keep them clean with filtered fuel, and treat water in the fuel as an emergency. No pump pressure or clearance figure is given here, because none is sourced.
Worked Example: Diesel And Petrol At The Same Ratio And At Their Own
Use γ = 1.4, a recalled value, and a cutoff ratio of 2, assumed for the arithmetic.
- Compression factor at r = 16: 16^0.4 = e^(0.4 × 2.7726) = e^1.109 = 3.031.
- Ideal air temperature at the top, from air at 20 °C (293.15 K): 293.15 × 3.031 = 888.7 K, which is 615.5 °C, about 616 °C. This agrees with the carrier’s 500+ °C.
- Cutoff term: (2^1.4 − 1) ÷ (1.4 × (2 − 1)) = (2.639 − 1) ÷ 1.4 = 1.171.
- Diesel efficiency: 1 − 1.171 ÷ 3.031 = 1 − 0.386 = 0.614, which is 61.4%.
- Petrol (Otto) at the same r = 16: 1 − 1 ÷ 3.031 = 0.670, which is 67.0%.
- Petrol at its own practical ratio of 8: 1 − 8^(−0.4) = 0.565, which is 56.5%.
At the same ratio the Otto cycle looks better on paper, but a petrol engine cannot run at 16 without knocking, while a diesel can. At its own ratio the diesel gives 61.4% against the petrol engine’s 56.5%. These are ideal figures; real engines of both kinds fall well below them.
Common Mistakes
- Water or dirt in the fuel. The pump and injector fits are tight and are damaged first.
- Expecting a diesel to start cold without help. Compression heat is lost to cold walls; salvaged engines come with a glow plug or heater for the purpose.
- Fuelling with doubtful vegetable oil. Cold, thick oil does not spray properly; preheat as the carrier says.
- Treating the engine as safe because it has no spark. It has hot parts, high pressures and fuel under pressure.
Safety
Fuel from an injector or a pipe under pressure can pierce skin
A fine jet of fuel under high pressure can enter the body through unbroken skin and cause serious injury (recalled hazard, flagged). Never check an injector line for leaks with a hand or a finger. Use cardboard or a rag held away and wear eye protection. Methanol used in making biodiesel is toxic, absorbed through skin and lungs, as the carrier warns. Run diesels outdoors; the exhaust carries carbon monoxide.
What’s Next
- Learn next: 2 vs 4 Stroke — how the cycle is packaged into strokes
- Combine with: Fuel Systems — where the fuel for either cycle comes from