UPHESC Assistant Professor Agricultural Engineering PYQs

Concept: Air-blown gasification gives producer gas: CO ≈ 18–22%, H₂ ≈ 15–20%, N₂ ≈ 45–50%, plus some CH₄ and CO₂.

1. CO and H₂ are the combustible parts; N₂ comes from the air and dilutes the gas.
2. Calorific value is low, about 4.5–5 MJ/m³.

Short trick: Producer gas = CO + H₂ + lots of N₂.

Concept: The sun (≈ 5800 K black body) radiates almost all its energy in the short-wave band of about 0.2–4.0 µm.

1. About 7% is UV (< 0.38 µm), 47% visible and 46% near-infrared.
2. Earth's long-wave emission lies at about 4–100 µm.

Short trick: Solar = short-wave 0.2–4 µm; terrestrial = long-wave > 4 µm.

Concept: Daily global radiation on a horizontal surface in India is about 14.5 MJ/m²/day in the winter months and rises to about 23 MJ/m²/day in summer (April–May).

1. Clear skies and a high sun in summer give the maximum.
2. Monsoon cloud lowers it in July–August.

Short trick: Winter ≈ 14.5, summer ≈ 23 MJ/m²/day.

Concept: In pressure-nozzle atomisation, concentrated milk is pumped through a fine nozzle at about 140–210 kg/cm² (2000–3000 psi).

1. This breaks it into fine droplets for rapid drying in hot air.
2. Centrifugal (spinning disc) atomisers are the other type and need no high pressure.

Short trick: Pressure nozzle ≈ 2000–3000 psi ≈ 140–210 kg/cm².

Concept: Pasteurisation (63 °C for 30 min, or HTST 72 °C for 15 s) destroys all pathogenic organisms and most spoilage organisms.

1. Fat and moisture are not removed.
2. It is designed around destroying Coxiella burnetii, the most heat-resistant pathogen in milk.

Short trick: Pasteurise → pathogens die, milk stays.

Concept: 1 TR is the heat removed to freeze 1 ton of water at 0 °C in 24 h, = 3.5 kW ≈ 211 kJ/min ≈ 3024 kcal/h.

1. 3024 kcal/h ÷ 60 ≈ 50 kcal/min.

Short trick: 1 TR ≈ 3.5 kW ≈ 50 kcal/min ≈ 12,000 BTU/h.

Concept: Homogenisation breaks fat globules from about 3–4 µm down to below 1 µm.

1. The number of globules and their total surface area rise many times.
2. More surfaces scatter more light, so the milk looks whiter (greater whitening power).

Short trick: Smaller globules → more fat surface → more light scattering.

Concept: Rennet (chymosin/rennin from the calf stomach) coagulates casein to form the curd in cheese making.

1. Pepsin and ficin can act as substitutes but give bitterness; trypsin is not used.
2. Microbial and fermentation-produced chymosin are modern alternatives.

Short trick: Cheese curd ← Rennet (chymosin).

Concept: Centrifugal force on a particle is $\frac{m{v}^2}{r}$, so separation improves with higher inlet velocity and smaller radius.

1. Increasing the inlet air velocity raises the centrifugal force → finer particles are caught (within limits of re-entrainment).
2. A larger cyclone diameter or a larger exit duct lowers efficiency.

Short trick: Fast and narrow spin = better separation.

Concept: The flow diversion valve (FDV), worked by the temperature recorder-controller, sends milk forward only if it has reached 72 °C.

1. Under-heated milk is diverted back to the balance tank for reheating.
2. The heat exchanger only heats or cools the milk; it does not route the flow.

Short trick: FDV = safety gate of HTST.

Concept: Registration: at each end of the stroke, the centre line of every knife section should coincide with the centre line of the guard.

1. Correct registration gives uniform cutting by every section and avoids ragged stubble.
2. It is adjusted by moving the pitman or cutter bar.

Short trick: Register = section centred on guard.

Concept: Diesel engines have a high thermal efficiency (about 30–35%), so their SFC is about 0.18–0.2 kg per bhp-h (≈ 250–270 g/kWh).

1. Petrol engines are less efficient, about 0.25–0.3 kg per bhp-h.
2. SFC = fuel used per hour ÷ brake power.

Short trick: Diesel ≈ 0.2, petrol ≈ 0.25–0.3 kg/bhp-h.

Concept: A tractor PTO (540 rpm) drives the rotavator through a reduction gearbox, so the rotor turns at about 150–300 rpm (commonly 175–250).

1. L- or C-shaped blades on the rotor cut and pulverise the soil in one pass.
2. 540 rpm is the PTO speed itself; 1500–3000 rpm is engine speed.

Short trick: PTO 540 → gearbox → rotor ~200 rpm.

Concept: The size of an MB plough is the width of the furrow cut by one bottom.

1. Animal-drawn MB ploughs are about 10–20 cm.
2. Tractor-drawn bottoms are commonly 30–40 cm (12–16 inch), e.g. a 2-bottom 35 cm plough.

Short trick: Bullock ≈ 15 cm, tractor ≈ 35 cm per bottom.

Concept: Spray classes: aerosol < 50 µm, mist 50–100, fine 100–200, medium 200–400, coarse > 400 µm.

1. Herbicides are applied as a coarse spray (> 400 µm) so that drift onto nearby crops is avoided.
2. Insecticides and fungicides use fine to medium droplets for better coverage.

Short trick: Herbicide = coarse, big drops (> 400 µm) → no drift.

Concept: Shaft power = $\frac{pQ}{\eta }$.

1. p = 35 kg/cm² = 35 × 9.81 × 10⁴ ≈ 3.43 × 10⁶ Pa.
2. Q = 25 L/min = 4.17 × 10⁻⁴ m³/s → hydraulic power ≈ 1.43 kW.
3. Shaft power = 1.43 ÷ 0.85 ≈ 1.68 kW ≈ 1.67 kW.

Short trick: Quick rule: kW = p(kg/cm²) × Q(L/min) ÷ (612 η) = 35 × 25 ÷ (612 × 0.85) ≈ 1.68.

Concept: Working across the slope on contours brings a risk of sideways overturning, so the slope limits safe tractor operation.

1. Wheeled tractors can work along contours on slopes up to about 16%.
2. On steeper land, terracing or animal or hand tools are used instead.

Short trick: Contour tractor work ≤ 16% slope.

Concept: Skid: kinetic energy = friction work, i.e. $\frac{1}{2}m{v}^2=\mu mgs$, so $v=\sqrt{2\mu gs}$.

1. $v=\sqrt{2\times 0.5\times 9.81\times 60}=\sqrt{588.6}=24.26$ m/s.
2. × 3.6 = 87.34 kmph.

Short trick: Skid length s = v²/(2μg).

Concept: Modern tractor engine pistons are cast from aluminium alloy (Al–Si).

1. They are light, so inertia forces are low, and their high thermal conductivity keeps the crown cool.
2. Cast iron was used in older or slow engines; steel is used only for rings and pins.

Short trick: Light + cool = aluminium alloy piston.

Concept: Cast iron is strong in compression, wears well and keeps its strength at high temperature.

1. Its coefficient of expansion is low, not high, so it allows a close fit.
2. It resists high temperature well; a material does not control engine speed.

Short trick: Cast iron: strong in compression, low expansion, wear-resistant.