Farm Power Sources and I.C. Engines: Cycles, Components, Systems, Fuels, Lubricants and Performance

An engine converts the chemical energy of fuel into work by burning it inside the working gas. Its ideal is an air-standard cycle: air as a perfect gas of constant specific heats, heat added and rejected reversibly, compression and expansion isentropic. The Otto cycle (spark-ignition, SI) adds heat at constant volume and has η = 1 − 1/r^(γ−1), depending only on the compression ratio r. The Diesel cycle (compression-ignition, CI) adds heat at constant pressure, with cut-off ratio ρ = V₃/V₂, and η = 1 − (1/r^(γ−1))·(ρ^γ − 1)/(γ(ρ − 1)); the bracket exceeds 1, so for the same r the Otto cycle is more efficient. The dual cycle adds heat partly at constant volume and partly at constant pressure and lies between them. CI engines are nevertheless more efficient in practice because they run at compression ratios roughly twice those of SI engines — an SI engine is limited by knock, a CI engine needs the high compression to ignite its fuel.

Worked: at r = 8 and γ = 1.4 the Otto efficiency is 1 − 8^(−0.4) = 1 − 0.435 = 56.5 per cent. At r = 16 with cut-off ratio 2, the Diesel efficiency is 1 − (1/16^0.4)(2^1.4 − 1)/(1.4 × 1) = 1 − 0.3299 × 1.1707 = 61.4 per cent — higher, because of the doubled compression ratio.

A four-stroke engine completes suction, compression, power and exhaust in two crankshaft revolutions, so it has one power stroke per cylinder every two revolutions and its camshaft turns at half crankshaft speed. A two-stroke engine uses ports and crankcase (or blower) scavenging to complete the cycle in one revolution, giving one power stroke per revolution and a higher power-to-weight ratio at the cost of scavenging losses and fuel economy.

The components are the cylinder block and head, piston with compression and oil rings, gudgeon pin, connecting rod, crankshaft with flywheel, camshaft, valves with their springs and rockers, and the timing gears. Around them sit the systems the syllabus names. Fuel: an SI engine mixes air and fuel in a carburettor or by port injection; a CI engine has a fuel tank, filters, a fuel-injection pump that meters and pressurises, and injectors that atomise fuel into the hot compressed air, with a governor controlling speed. Cooling: air cooling by fins, or water cooling with a pump, radiator, fan and a thermostat that keeps the engine at working temperature. Lubrication: splash, or forced-feed from a gear pump through filters to the bearings. Ignition: battery-coil or magneto for SI engines (a magneto generates its own current and needs no battery); CI engines need no ignition system. Electrical: battery, alternator or dynamo, starter motor, lights. Intake and exhaust: an air cleaner (oil-bath or dry paper element, vital in dusty field work), the manifolds, a silencer, and on many engines a turbocharger.

Lubricants separate moving surfaces, cool, seal and clean. Their properties: viscosity (resistance to flow) and viscosity index (how little the viscosity changes with temperature — high VI is better); flash point and fire point (safety); pour point (the lowest temperature at which the oil still flows); oiliness and detergency. SAE grades classify viscosity; a multigrade oil such as 20W-40 behaves like a thin SAE 20 oil when cold (the W is for winter) and like an SAE 40 oil when hot, thanks to viscosity-index improvers.

Indicated power is the power developed by the gas on the pistons: IP = p_mi·L·A·n·k/60, with p_mi the indicated mean effective pressure (Pa), L the stroke (m), A the piston area (m²), k the number of cylinders, and n the number of power strokes per minute per cylinder — N/2 for a four-stroke engine and N for a two-stroke. Brake power, measured at the crankshaft by a dynamometer, is BP = 2πNT/60. Friction power FP = IP − BP is lost in rubbing, pumping and driving accessories, and mechanical efficiency is η_m = BP/IP. Friction power is found by the Morse test (for multi-cylinder engines, cutting out one cylinder at a time at constant speed and measuring the fall in BP), by Willan’s line (for CI engines, extrapolating fuel consumption against BP back to zero fuel), or by motoring the engine.

Brake thermal efficiency is η_bth = BP/(ṁ_f × CV), and brake specific fuel consumption is bsfc = ṁ_f/BP, usually in kg/kWh; the two are reciprocal apart from the calorific value, since η_bth = 3600/(bsfc × CV) with CV in kJ/kg. An engine burning 5 kg/h of diesel of CV 42 000 kJ/kg (58.3 kW of fuel energy) to give 20 kW has bsfc = 0.25 kg/kWh and η_bth = 34.3 per cent. The heat balance accounts for the rest of the fuel energy: heat to the brake power, to the cooling water, to the exhaust gases, and an unaccounted remainder (radiation and the like). Volumetric efficiency compares the air actually inducted with the swept volume at intake conditions.

Performance curves plot torque, brake power and bsfc against speed at full throttle (or full rack). Torque peaks at a lower speed than power, because power keeps rising with speed after torque has begun to fall; bsfc is lowest near the speed of maximum torque. The torque backup (torque reserve) = (T_max − T_rated)/T_rated × 100 measures how well an engine lugs through a sudden load without stalling — a tractor engine with a rated torque of 150 N·m and a peak of 180 N·m has a 20 per cent backup. A governor holds speed against load changes, which is why a tractor engine at part load runs along a steep governor line rather than the full-throttle curve.

An engine is selected for the load it drives: the power the job needs at the shaft, the speed range, the duty (continuous for a pump, fluctuating for a thresher), the fuel available, and the ambient conditions. Engines lose power at altitude and in heat because the air is thinner, so a rating is derated for site conditions. For a stationary pump set, a CI engine is chosen for its fuel economy and long life; for light portable jobs, a small SI engine for its weight and ease of starting.

• Daily: check fuel, lubricating oil and coolant levels; clean or check the air cleaner in dusty work; drain water from the fuel filter.
• Periodic: change engine oil and oil filter, replace fuel filters, check fan-belt tension, service the injectors and set valve clearances at the hours the manufacturer specifies.
• Symptoms and causes: black smoke — overload, a choked air cleaner or faulty injection; blue smoke — oil burning past worn rings or guides; white smoke — water in the fuel or a cold engine; overheating — low coolant, a slipping fan belt, a stuck thermostat or a scaled radiator.
• Overhaul: when compression, oil consumption and blow-by show worn rings, liners and bearings — reboring or relining, new rings and bearings, valve grinding.