Agricultural Power, Machinery, and Allied Subjects Study Notes

Authorial Background and Scope of work

Alexis T. Belonio is a Professional Agricultural Engineer and researcher who served as an Associate Professor and Chairman of the Department of Agricultural Engineering and Environmental Management at Central Philippine University (CPU) in Iloilo City. He holds both Bachelor and Master of Science degrees in Agricultural Engineering from Central Luzon State University. His professional experience, which began in 1983, spans teaching, research, and project development. Recognitions include being named the Most Outstanding Agricultural Engineer by the Philippine Society of Agricultural Engineers (PSAE) and Outstanding Professional by the Professional Regulation Commission (PRC) in 1993, and an Outstanding Young Filipino award in 1997. The Comprehensive Board Exam Reviewer Volume I, published in 2004 by CPU Press, is the first of a three-volume series intended for graduates preparing for the Professional Agricultural Engineering Licensure Examination. Volume I specifically addresses agricultural power, machinery, production and post-production equipment, design, manufacturing, mechanization, and management aspects.

Thermodynamics and Internal Combustion Engines

Internal combustion engines are classified by their technical operating cycles, primarily the Otto cycle and the Diesel cycle. The Otto cycle is characterized as a constant volume combustion cycle, predominantly used in gasoline engines where a spark plug provides ignition to a harmonious mixture of fuel and air metered by a carburetor. In contrast, the Diesel cycle is a constant pressure combustion cycle used in compression-ignition engines. In a diesel engine, fuel is injected into the combustion chamber near the end of the compression stroke and is ignited solely by the heat of highly compressed air. Air-fuel ratios for gasoline engines range from $15:1$ for standard operation to $11.0–12.5:1$ under idling or full-load conditions.

The mechanical operation of these engines occurs in either two-stroke or four-stroke cycles. A two-stroke cycle engine produces power in every single revolution of the crankshaft, while a four-stroke cycle engine requires two revolutions (or four piston strokes) for every power stroke. The sequence of strokes in a four-stroke engine is intake, compression, power, and exhaust. The power developed by the gas within the cylinders is termed the Indicated Horsepower ($IHP$), while the usable power delivered at the crankshaft is the Brake Horsepower ($BHP$). The ratio of $BHP$ to $IHP$ constitutes the mechanical efficiency of the engine. Firing orders vary by engine configuration; for example, a common firing order for a four-cylinder engine is $1-3-4-2$, whereas a six-cylinder straight engine may follow a $1-5-3-6-2-4$ sequence.

Engine maintenance and performance are measured through various parameters and diagnostic tools. Compression ratio is defined by the formula $CR = \frac{V_D + V_C}{V_C}$, where $V_D$ is the piston displacement volume and $V_C$ is the clearance volume. Piston displacement is the volume displaced by the piston as it moves between Top Dead Center (TDC) and Bottom Dead Center (BDC). Common engine troubles include knocking or detonation, often caused by incorrect ignition timing, poor fuel quality, or carbon buildup. For instance, if gasoline engine timing is too far advanced, the spark plug ignites the fuel while the piston is still completing its compression stroke. Maintenance tools include the hydrometer for battery charge, the tachometer for shaft speed, and the dynamometer for measuring power performance.

Power Transmission Systems

Power transmission in agricultural machinery utilizes direct drives, belts, chains, and gears. A direct drive occurs when a machine is driven directly from the shaft of its power source. For belt transmissions, agricultural V-belts are designated by sizes HA, HB, HC, HD, and HE, which differ from standard industrial A, B, C, D, and E belts. The speed and torque transmission between two pulleys are determined by their diameter ratios; for instance, if an engine running at $2500\text{ rpm}$ drives a pump at $1000\text{ rpm}$ using a $4\text{-inch}$ pump pulley, the engine pulley must be $1.6\text{ inches}$ ($10\text{ in.}$ pulley calculation depending on setup specifics). The length of a flat-belt drive is calculated based on the diameters of both pulleys and the center distance between their shafts.

Chain drives, such as those using ANSI roller chains (e.g., RC-40, RC-50, RC-60), are used for power transmission over medium distances. The strength of a chain increases as its number designation increases. For sprocket selection, high-speed operations typically require a minimum of $18–24$ teeth to ensure smooth operation. Gear systems are used for high-efficiency power transmission when shafts are close together or intersecting. Spur gears have straight teeth parallel to the shaft axis and are common in agricultural gearboxes. Bevel gears are used for intersecting shafts, usually at a $90\text{-degree}$ angle, while worm gears are common in gear reducers and gear motors. Power transmission components also include the clutch, which is a gripping device used to connect and disconnect power input, and universal joints, which transmit torque between intersecting shafts where the angle is variable.

Tractive Power and Agricultural Tractors

Agricultural tractors are mobile power units designed to pull, propel, and supply power to machinery. They are equipped with a Power Take-Off (PTO) shaft, which provides rotational power to implements. The standard PTO speed is $540\text{ rpm}$, though $1000\text{ rpm}$ is also common for modern equipment. The PTO shaft profile typically features either $6$, $20$, or $21$ splines depending on the diameter (typically $35\text{ mm}$ or $45\text{ mm}$) and speed rating. For traction, tractors utilize ballast (added weight) to improve stability and wheel grip. Tractive efficiency is defined as the ratio of drawbar power to wheel axle power. Slip is a common occurrence on loose soil; allowable wheel slip for maximum efficiency is generally $10–15$ for two-wheel drive tractors on firm soil and $6–10$ for four-wheel drive tractors.

Power tillers, or hand tractors, are two-wheeled units used primarily as a substitute for draft animals. A puddling-type floating tiller is specialized for field conditions soaked for at least half a day to soften the soil, operating at an optimum axle speed of approximately $250\text{ rpm}$ ($200–250\text{ rpm}$ range). Modern tractors feature Automatic Draft Control, a hydraulic hitch system that automatically raises or lowers soil-engaging implements to maintain a pre-selected draft value. The center of gravity for a two-wheel drive tractor is typically located about $1/3$ of the wheelbase ahead of the rear axle.

Tillage and Land Preparation

Tillage is the mechanical manipulation of soil to provide a favorable environment for crop growth. Primary tillage equipment, such as moldboard plows, disk plows, and rotary plows, is used to break, displace, and shatter the soil. Moldboard plows consist of a share (for cutting), a shin, and a moldboard (for turning and pulverizing the furrow slice). A landside is used to counteract the side pressure exerted by the furrow slice. Disk plows use large concave disks and are valued for their ability to work in hard, dry, or stony soils where moldboard plows struggle. The disk angle, typically $40–45\text{ degrees}$ relative to the direction of travel, and the tilt angle (relative to the vertical plane) influence the plow's penetration and draft.

Secondary tillage operations, such as harrowing and pulverizing, follow primary tillage to refine the seedbed and control weeds. Disk harrows can be arranged in single-action, double-action (tandem), or offset configurations. An offset disk harrow is unique because it is not centrally located relative to the tractor. Other tillage methods include minimum tillage (limiting soil manipulation), zero-tillage (planting directly into undisturbed soil), and subsoiling (deep chiseling below $16\text{ inches}$ to break up plow pan or improve drainage). The draft of a plow, measured in force per unit area of tilled cross-section, is influenced by soil type, moisture content, depth of cut, and speed of operation.

Planting, Seeding, and Crop Protection

Planters and seeders are designed to place seeds or plant parts into the soil. A drill seeder deposits seeds continuously in rows, while a hill-drop planter places groups of seeds at specific intervals. A check-row planter enables hill planting at equal distances in both directions to allow for cross-cultivation. Precision planting involves depositing single seeds at controlled intervals. The number of plants per hectare depends on spacing; for example, a $2\text{-hectare}$ farm with $25\text{ cm} \times 45\text{ cm}$ spacing would require approximately $177,777$ plants. Seeder calibration is the process of adjusting the application rate to ensure the desired population density.

Crop protection is achieved through sprayers and dusters. Sprayers regulate the application of chemicals to avoid waste and harm. Knapsack sprayers are portable units carried on the back, while boom sprayers employ an over-the-crop boom for uniform large-area coverage. Effective spraying depends on droplet size, which is controlled by pressure and nozzle type; higher pressure generally produces smaller droplets. Weeding index refers to the ratio of weeds removed to the initial weed population per unit area. Cultivation practices include barring-off (throwing soil away from plants) and hilling-up (throwing soil toward plants).

Harvesting and Post-Harvest Machinery

Harvesting machines like reapers cut stalks and place them in windrows, while combine harvesters gathered, cut, threshed, and cleaned grain in one operation. A stripper harvester specifically combs grain from the plant while the plant remains anchored. Threshing is the process of detaching grain from the panicle or cob. Axial-flow threshers move material in a helical path around a threshing cylinder, typically using peg-teeth for separation. Threshing efficiency is the ratio of clean threshed grain to total grain input. Mechanical grain dryers use heated air to reduce moisture content for storage; burner or furnace efficiency measures the heat released by the fuel compared to the theoretical heat available. Professional standards, such as those from the Philippine Agricultural Engineering Standards (PAES), dictate testing protocols for these machines, including grain-straw ratios of $0.5:1$ to $0.65:1$ for thresher evaluations.

Alternative Energy and Management

Agricultural power is also derived from renewable sources. Wind power is generated from moving air, with power output proportional to the cube of the wind speed and the square of the rotor diameter. Solar power involves converting sunlight into electricity via photovoltaic (PV) panels. Biomass energy can be captured through combustion in stoves or gasification. Gasification converts solid fuels into producer gas (primarily carbon monoxide, hydrogen, and methane) through thermochemical reactions. Digesters produce biogas (methane and carbon dioxide) from organic materials like manure through anaerobic digestion; the ideal carbon-to-nitrogen ($C:N$) ratio for this process is between $20:1$ and $30:1$. General management of agricultural machinery involves calculating the Theoretical Field Capacity ($TFC$), Effective Field Capacity ($EFC$), and field efficiency. $TFC$ is the rate of performance if a machine uses $100$ of its width at its rated speed without interruptions, while $EFC$ accounts for time losses like turning and repairs.