GATE Agricultural Engineering Farm Power Syllabus

Which thermodynamic cycle is the air-standard cycle for a petrol (spark-ignition) engine, and what process represents its heat addition?

The Otto cycle. Heat addition occurs at constant volume (the constant-volume combustion process).

Which air-standard cycle represents the ideal slow-speed diesel (compression-ignition) engine, and how is heat added?

The Diesel cycle. Heat is added at constant pressure during combustion.

Name the air-standard cycle that combines constant-volume and constant-pressure heat addition, used for modern high-speed diesel engines.

The Dual (or limited-pressure / mixed) cycle, in which part of the heat is added at constant volume and part at constant pressure.

Write the expression for the air-standard efficiency of the Otto cycle.

$$\eta_{Otto} = 1 - \frac{1}{r^{\gamma-1}}$$ where $r$ is the compression ratio and $\gamma$ is the ratio of specific heats ($\gamma = \frac{c_p}{c_v}$).

Write the air-standard efficiency of the Diesel cycle.

$$\eta_{Diesel} = 1 - \frac{1}{r^{\gamma-1}}\left[\frac{\rho^{\gamma}-1}{\gamma(\rho-1)}\right]$$ where $r$ is compression ratio, $\rho$ is the cut-off ratio, and $\gamma$ is the specific-heat ratio.

For the same compression ratio, which has higher air-standard efficiency: the Otto or the Diesel cycle, and why?

For the same compression ratio, the Otto cycle has higher efficiency than the Diesel cycle, because the Diesel-cycle efficiency term in brackets $\frac{\rho^{\gamma}-1}{\gamma(\rho-1)}$ is always greater than 1, reducing its efficiency.

Define compression ratio of an I.C. engine and give typical ranges for petrol and diesel engines.

Compression ratio $r = \dfrac{V_{total}}{V_{clearance}} = \dfrac{V_s + V_c}{V_c}$. Petrol (SI) engines: about $6{:}1$ to $11{:}1$; diesel (CI) engines: about $14{:}1$ to $22{:}1$.

Define swept (displacement) volume of a cylinder and give its formula.

Swept volume is the volume displaced by the piston between TDC and BDC: $$V_s = \frac{\pi}{4} D^{2} L$$ where $D$ is bore (cylinder diameter) and $L$ is stroke length.

Define indicated power (IP) of an engine and give its formula.

Indicated power is the power developed inside the cylinder by combustion gases. $$IP = \frac{p_m\, L\, A\, N\, n}{60}$$ where $p_m$ = mean effective pressure, $L$ = stroke, $A$ = piston area, $N$ = working strokes per minute, $n$ = number of cylinders.

Define brake power (BP) and give the formula using a dynamometer.

Brake power is the useful power available at the engine output shaft. $$BP = \frac{2\pi N T}{60}$$ where $N$ is speed in rpm and $T$ is torque in $\text{N·m}$ ($BP$ in watts).

Define friction power and write its relation to IP and BP.

Friction power (FP) is the power lost in overcoming internal friction and driving auxiliaries: $$FP = IP - BP$$

Define mechanical efficiency of an I.C. engine.

$$\eta_{mech} = \frac{BP}{IP}$$ the ratio of brake power (useful output) to indicated power (power developed in the cylinder).

Define brake thermal efficiency of an engine.

$$\eta_{bth} = \frac{BP}{\dot{m}_f \times CV}$$ the ratio of brake power to the rate of energy supplied by fuel, where $\dot{m}_f$ is fuel consumption rate and $CV$ is the calorific value of the fuel.

Define brake specific fuel consumption (BSFC).

$$BSFC = \frac{\dot{m}_f}{BP}$$ the mass of fuel consumed per unit brake power per unit time (e.g. $\text{kg/kW·h}$). Lower BSFC means a more fuel-efficient engine.

What is mean effective pressure (MEP) of an engine?

Mean effective pressure is the constant (average) pressure which, acting on the piston throughout the power stroke, would produce the same work as the actual variable cylinder pressure. It is work per cycle divided by swept volume.

Name the main stationary and moving components of an I.C. engine.

Stationary parts: cylinder block, cylinder head, crankcase. Moving parts: piston, piston rings, connecting rod, crankshaft, camshaft, valves, flywheel. Also gudgeon (piston) pin and timing gears.

What is the function of the flywheel in an I.C. engine?

The flywheel stores rotational energy during the power stroke and releases it during the idle (non-power) strokes, smoothing out speed fluctuations and ensuring uniform crankshaft rotation.

What is the function of piston rings, and name their two types?

Piston rings seal the gap between piston and cylinder wall. Compression rings (upper) prevent gas leakage/blow-by; oil control rings (lower) scrape excess oil off the cylinder wall and return it to the sump.

What is the function of the camshaft, and at what speed does it rotate relative to the crankshaft in a 4-stroke engine?

The camshaft operates the intake and exhaust valves (via cams, tappets, push-rods, rockers) at the correct timing. In a four-stroke engine it rotates at half ($\frac{1}{2}$) the crankshaft speed.

Differentiate between calorific value (CV) of a fuel and give units.

Calorific value is the heat energy released by complete combustion of unit mass (or volume) of fuel, expressed in $\text{kJ/kg}$ (or $\text{MJ/kg}$). It is divided into higher (gross) CV and lower (net) CV.

Distinguish higher calorific value (HCV) from lower calorific value (LCV).

HCV includes the latent heat of the water vapour formed during combustion (water condensed). LCV excludes this latent heat: $$LCV = HCV - m_w \times h_{fg}$$ where $m_w$ is mass of water formed per kg fuel and $h_{fg}$ is latent heat of vaporisation.

What property of petrol indicates its resistance to knock, and how is it measured?

The octane number indicates a petrol's resistance to knocking (auto-ignition). It is defined on a scale where iso-octane = 100 and n-heptane = 0; higher octane number means greater knock resistance.

What property of diesel indicates its ignition quality, and what does a higher value mean?

The cetane number indicates diesel ignition quality. A higher cetane number means shorter ignition delay and easier auto-ignition; cetane (n-hexadecane) = 100 and alpha-methylnaphthalene = 0 on the scale.

Define the air–fuel ratio and give the approximate stoichiometric air–fuel ratio for petrol.

Air–fuel ratio is the mass ratio of air to fuel in the combustible mixture. The stoichiometric (chemically correct) ratio for petrol is about $15{:}1$ (roughly 14.7:1 by mass).

What is meant by 'knocking' (detonation) in a spark-ignition engine?

Knocking is the spontaneous auto-ignition of the end gas (last portion of the unburnt charge) ahead of the flame front, causing a sudden pressure rise and a metallic knocking sound. It reduces efficiency and can damage the engine.

Name the desirable properties of a good lubricant.

Suitable viscosity, high viscosity index (small change of viscosity with temperature), good oiliness, high flash and fire point, low pour point, oxidation/corrosion resistance, and thermal/chemical stability.

Define viscosity index of a lubricating oil.

Viscosity index (VI) is a number expressing how much an oil's viscosity changes with temperature. A high VI means the viscosity changes little with temperature (desirable); a low VI means large change.

Differentiate flash point and fire point of a lubricant.

Flash point is the lowest temperature at which oil vapour momentarily flashes/ignites on application of a flame but does not sustain. Fire point is the temperature at which the oil burns continuously for at least 5 seconds; it is slightly higher than the flash point.

Name the three main systems of an I.C. engine besides ignition, as listed in farm-power studies.

The fuel (supply) system, the cooling system, and the lubrication system (along with the ignition/fuel-injection system).

What are the two main methods of cooling an I.C. engine, and which is used in tractors?

Air cooling (fins on cylinder, used in small engines/power tillers) and liquid/water cooling (radiator, water jacket, pump, fan, thermostat — used in most tractors and larger engines).

Name the common methods of engine lubrication and the system used in most modern tractor engines.

Methods: splash lubrication, pressure (forced-feed) lubrication, and combination (splash + pressure) systems. Most modern tractor engines use the forced-feed (pressure) lubrication system driven by an oil pump.