Engines and Combustion Devices: Power, Losses, and Practical Optimization

What “power” means in real engines and combustion devices

In maker projects, “engine” can mean anything that turns chemical energy into shaft power (a small gasoline generator, a lawnmower engine, a kart engine), or into hot gas flow (a propane burner, a rocket-style combustor, a DIY forge). “Combustion device” also includes heaters and boilers where the goal is heat output rather than rotation. In all of these, you care about three practical numbers: output power (what you actually get), input fuel rate (what you pay for), and losses (where performance disappears).

For shaft-producing engines, output power is mechanical power at the crankshaft or at the generator terminals. For heat-producing devices, output is useful heat delivered to a load (water, air, metal workpiece). A common maker mistake is to compare devices using only fuel consumption or only “rated horsepower” without checking the conditions (speed, load, ambient temperature, altitude, fuel type, and measurement method). Practical optimization starts by measuring what you can and separating “useful output” from “waste paths.”

Key performance metrics you can actually measure

• Brake power (shaft power): measured with a dynamometer, a prony brake, or inferred from generator electrical output plus generator efficiency.
• Indicated power: power produced in the cylinder before mechanical friction; usually not directly measured by makers, but it explains why friction and pumping losses matter.
• Thermal efficiency: useful output divided by fuel chemical energy rate. For engines, this is brake thermal efficiency; for heaters, it is delivered heat divided by fuel energy.
• Specific fuel consumption: fuel mass (or volume) per unit energy output (e.g., g/kWh). This is a practical “how thirsty is it” number.
• Air–fuel ratio (AFR) and equivalence ratio: how rich/lean the mixture is relative to stoichiometric. This strongly affects power, temperature, emissions, and component life.

Combustion basics that matter for optimization (without re-teaching cycles)

Combustion is a fast chemical reaction that releases heat. In engines and burners, that heat raises gas temperature, which raises pressure or creates high-velocity flow. The device then converts part of that energy into useful work or useful heat transfer. The details of the thermodynamic cycle were covered earlier; here the focus is on what limits real devices and what knobs you can turn safely.

Stoichiometric, lean, and rich: why “perfect” is not always best

Stoichiometric mixture is the chemically “just enough oxygen” point. In practice:

• Maximum power in spark-ignition engines often occurs slightly rich of stoichiometric because extra fuel can increase burn speed and reduce knock tendency by cooling the charge (via evaporation) and changing flame temperature.
• Best fuel economy often occurs slightly lean (if the engine can run stable and avoid misfire), because pumping losses can be reduced and less fuel is used for the same load.
• Lowest exhaust temperature can occur rich (evaporative cooling and incomplete combustion) or very lean (lower heat release rate), but both can create other issues (carbon deposits rich; misfire lean).

For burners and heaters, running slightly lean is common to reduce soot and carbon monoxide, but too lean can cause flame instability and blow-off. For forges, a slightly rich flame is sometimes used to create a reducing atmosphere to protect steel from oxidation, but it wastes fuel and increases CO risk.

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Combustion speed, ignition timing, and knock (spark-ignition)

In a spark-ignition engine, the spark must occur before peak pressure is desired because the flame takes time to propagate. If ignition is too late, peak pressure occurs too far after top dead center and torque drops while exhaust temperature rises. If ignition is too early, pressure rises while the piston is still compressing, increasing negative work and heating, and it can trigger knock.

Knock is uncontrolled auto-ignition of end-gas, producing pressure waves that can damage pistons and bearings. Makers often encounter knock when increasing compression, boosting, or running low-octane fuel. Practical knock control knobs include: reducing spark advance, enriching mixture slightly, lowering intake air temperature, reducing boost, improving intercooling, and using higher-octane fuel.

Diesel-style combustion: mixing-limited and timing-sensitive

Compression-ignition engines rely on fuel injection into hot compressed air. The key practical idea is that combustion is often limited by how well fuel mixes with air. Injection timing, spray pattern, and air motion (swirl/tumble) determine smoke, efficiency, and noise. Advancing injection can raise efficiency but increases peak pressure and NOx; retarding reduces NOx but raises exhaust temperature and soot. For makers working with small diesels, the safest “optimization” is usually maintenance: injector health, clean air filter, correct valve lash, and avoiding over-fueling.

Where the energy goes: a loss map you can use

When fuel energy enters an engine, it leaves through several channels. Thinking in channels helps you decide what to measure and what to change.

1) Exhaust losses

Hot exhaust carries a large fraction of energy away. High exhaust temperature can mean high load (normal) but can also signal late combustion timing, overly rich mixture, restricted exhaust, or cooling system problems. Exhaust energy is why turbochargers work: they recover some exhaust enthalpy to compress intake air. For heaters, exhaust losses show up as hot flue gases leaving the chimney; improving heat exchanger area or adding recuperation can raise delivered heat.

2) Cooling losses

Engines dump heat into coolant and oil. Some cooling is necessary to protect materials and control clearances, but excessive cooling losses reduce efficiency. Common causes include running too cold (stuck-open thermostat), overly rich mixture washing cylinder walls, or poor combustion causing heat to transfer to walls rather than expand the gas. In air-cooled engines, blocked fins and missing shrouds can cause hot spots and force you to run richer or with less timing, indirectly reducing efficiency.

3) Mechanical friction

Friction in rings, bearings, valve train, and accessories converts work into heat. Friction rises with speed and with oil viscosity when cold. Practical levers: correct oil grade, warm-up strategy, proper clearances, and avoiding unnecessary high RPM. For small engines, a surprising amount of “lost power” is simply running at a speed where friction and pumping dominate.

4) Pumping losses (breathing work)

Engines spend work pulling air past restrictions and pushing exhaust out. Throttled spark-ignition engines have significant pumping loss at part load because the throttle creates a pressure drop. Practical levers: reduce intake restriction (clean filter, smoother intake), reduce exhaust restriction (muffler design within noise limits), and for some applications choose an engine that can run closer to wide-open throttle at the desired power (gearing changes, different displacement).

5) Incomplete combustion and chemical losses

Unburned hydrocarbons, CO, and soot represent fuel that did not fully release its energy. Causes include overly rich operation, poor mixing, weak ignition, low compression, and cold walls (short trips, cold start). In burners, incomplete combustion often comes from insufficient air, poor burner geometry, or inadequate residence time.

Practical measurement: building a simple test plan

Optimization without measurement often becomes guesswork. You can do useful work with modest tools if you plan your measurements and keep conditions consistent.

Step-by-step: baseline an engine or burner before changing anything

• Step 1: Define the output you care about. Shaft power at a given RPM? Electrical power from a generator? Heat delivered to water? Pick one primary output metric.
• Step 2: Measure fuel input rate. For liquid fuel, weigh a small tank on a scale and time the run; for gaseous fuel (propane), weigh the cylinder before/after a timed test. Convert to energy using the fuel’s lower heating value if you have it; if not, use consistent “per gram” comparisons for tuning.
• Step 3: Measure operating conditions. Ambient temperature, barometric pressure/altitude, and humidity affect air density and mixture. Record them so you can compare runs.
• Step 4: Measure temperatures that indicate losses. Exhaust gas temperature (EGT) near the exhaust port (or flue temperature for burners), coolant temperature (if liquid-cooled), and oil temperature if available.
• Step 5: Measure output. For generators, measure voltage, current, power factor (if AC), and compute real power. For shaft engines, use a brake or dyno. For heaters, measure water flow rate and inlet/outlet temperatures to compute delivered heat rate.
• Step 6: Repeat at a few steady points. Choose 3–5 operating points (idle, mid-load, high-load). Hold each point long enough for temperatures to stabilize.

Simple heater output measurement (water as a calorimeter)

If you are optimizing a burner or boiler, water heating is a straightforward way to quantify useful heat. Measure water mass flow (or batch mass), and temperature rise across the heater. Useful heat rate is approximately:

Q_dot_useful ≈ m_dot_water * c_p_water * (T_out - T_in)

Keep the setup consistent: same flow rate, same inlet temperature, same insulation, and steady flame settings. Then compare fuel mass used per unit of delivered heat.

Optimization knobs for small engines (spark-ignition)

Small gasoline engines are common in maker builds (generators, pumps, mini vehicles). They are also sensitive to mixture, ignition, and airflow. The goal is usually one of: more power, lower fuel use at a fixed power, or better reliability at continuous load.

Step-by-step: mixture tuning with plug reading and EGT (carbureted engines)

• Step 1: Ensure mechanical health first. Valve lash, compression, clean air filter, no vacuum leaks, correct spark plug heat range, and fresh fuel. Tuning cannot fix a leaking intake or weak ignition.
• Step 2: Set a safe baseline jetting. Start slightly rich rather than lean to avoid overheating and detonation under load.
• Step 3: Choose a steady load point. For a generator, use a resistive load bank or known appliances to hold a constant electrical load.
• Step 4: Adjust main jet (or needle) in small steps. After each change, run long enough to stabilize EGT and head temperature.
• Step 5: Watch for lean symptoms. Surging, misfire, rising EGT with falling power, or a very light/white plug insulator can indicate too lean. Back off to a richer setting.
• Step 6: Confirm at multiple loads. A setting that works at mid-load may be too lean at full load. Verify the highest-load condition you expect.

EGT is a helpful indicator but not a universal “target number” because sensor placement and engine design vary. Use EGT trends: if a jet change increases power and reduces EGT slightly, you may have been too rich; if EGT climbs sharply while power drops, you are likely too lean or too retarded on timing.

Ignition timing adjustments: power vs temperature vs knock margin

If timing is adjustable, change it cautiously and only after mixture is reasonable. Practical method:

• Step 1: Mark the current setting and record baseline power/fuel/EGT.
• Step 2: Advance timing slightly (small angle change), repeat the same steady test point.
• Step 3: Stop advancing when power no longer increases, or when knock appears, or when temperatures rise unacceptably.
• Step 4: Back off to the last safe setting with stable operation.

For fixed-timing small engines, “optimization” is often about keeping the cooling system and mixture correct so the factory timing remains safe under your load profile.

Breathing improvements that actually help

• Intake: A less restrictive filter and smoother intake path can reduce pumping loss, but avoid removing filtration in dusty environments; abrasive dust can destroy rings quickly.
• Exhaust: Reducing backpressure can improve high-RPM power, but too little backpressure is not inherently harmful; the real risk is excessive noise and poor scavenging if the exhaust is badly tuned. Ensure exhaust routing does not overheat nearby parts.
• Gearing and operating point: Often the biggest “efficiency mod” is running the engine at a lower RPM for the same output by changing gearing or pulley ratios, if the engine can make the torque without lugging.

Optimization knobs for burners, forges, and boilers

For combustion devices where the goal is heat, optimization is usually about complete combustion, good heat transfer, and controlled exhaust losses.

Air supply and mixing: the foundation of clean, efficient heat

Burners need enough oxygen and enough mixing time. Common maker setups include naturally aspirated (venturi) propane burners and forced-air burners. Forced-air burners give you independent control of fuel and air, which makes tuning easier and can reduce CO when adjusted correctly.

Step-by-step: tuning a forced-air propane burner for stable, low-CO operation

• Step 1: Start with excess air. Set airflow moderately high and fuel low; ignite safely and confirm stable flame.
• Step 2: Increase fuel to reach desired heat output. Watch flame stability and color; avoid roaring instability or flame lift-off.
• Step 3: Reduce air gradually until efficiency improves. Signs of too much air include unnecessary high flue flow and lower chamber temperature. Signs of too little air include yellow sooty flame, smell, eye irritation, and CO risk.
• Step 4: Verify with measurement if possible. A combustion analyzer (O2/CO) is ideal. If you do not have one, prioritize safety: keep ventilation high and avoid operating in enclosed spaces.
• Step 5: Lock settings and re-check after warm-up. Burner behavior changes as the chamber heats; re-tune at operating temperature.

Heat exchanger effectiveness: getting heat into the load instead of the flue

In boilers and heaters, you want hot gases to transfer heat to water/air/metal before exiting. Practical levers:

• Increase surface area (fins, longer path, more tubes) while keeping flow resistance manageable.
• Create turbulence on the gas side to improve convection, but avoid soot-prone designs that become insulating over time.
• Insulate the hot side where heat is not useful (combustion chamber walls in a forge) to raise internal temperature and reduce fuel use for a given workpiece heating rate.
• Control excess air because too much air increases flue mass flow and carries heat away.

Turbocharging and waste heat recovery: when it’s worth it

Turbocharging uses exhaust energy to compress intake air, increasing the mass of air (and therefore fuel) the engine can burn, raising power density. For makers, turbocharging is attractive but adds thermal and mechanical stress. Practical considerations include: stronger fueling control, intercooling, knock margin (spark engines), exhaust temperature limits, and oil supply/return for the turbo.

Waste heat recovery in small builds is usually more practical as cogeneration: use engine waste heat to warm water or air while also producing power. A generator running at steady load can heat a shop via a coolant heat exchanger and an exhaust heat exchanger (with careful design to avoid exhaust leaks into occupied spaces). The optimization target becomes total useful energy (electric + heat) rather than electrical efficiency alone.

Reliability-focused optimization: power you can sustain

Many maker projects fail not because peak power is too low, but because continuous operation overheats components or causes rapid wear. Practical optimization includes:

• Cooling capacity margin: ensure airflow over fins, correct fan shrouds, clean radiators, and proper coolant mix.
• Lubrication control: maintain oil level, choose oil viscosity for expected ambient temperature, and consider oil cooling for continuous high load.
• Detonation margin: avoid borderline lean mixtures at high load, keep intake temperatures reasonable, and do not over-advance timing.
• Exhaust management: avoid excessive backpressure and keep exhaust components from heating fuel lines, wiring, and plastics.
• Vibration and mounting: misalignment and vibration waste power and break parts; use proper mounts, couplers, and balanced rotating components.

Common failure modes tied to thermodynamic causes (and what to check)

Overheating under load

• Thermodynamic cause: too much heat release for the available cooling and exhaust heat rejection, or combustion occurring too late (high EGT) or too early (high cylinder/head heating).
• Checks: mixture (lean can overheat), ignition timing, cooling airflow, clogged fins/radiator, low oil, restricted exhaust, and load higher than rated.

Low power and high fuel consumption

• Thermodynamic cause: incomplete combustion, excessive pumping loss, or high friction; energy is going to exhaust heat and losses instead of useful output.
• Checks: air filter restriction, carb/injector issues, weak spark, low compression, dragging bearings, incorrect governor setting, and muffler blockage.

Soot, smoke, or carbon deposits

• Thermodynamic cause: rich mixture and/or poor mixing leading to locally fuel-rich zones that form soot; low combustion temperature regions that fail to oxidize soot.
• Checks: air supply, burner alignment, injector spray pattern, choke stuck, and short-run cold operation.

Practical optimization examples

Example 1: Improving a small generator’s fuel use at a fixed electrical load

• Goal: minimize fuel per kWh at 1 kW output.
• Method: measure fuel mass over 20 minutes at steady 1 kW; record RPM, EGT, and head temperature.
• Changes to try: ensure clean air filter; verify governor holds correct RPM; adjust mixture slightly leaner only if temperatures remain safe and power stays stable; reduce unnecessary RPM if the generator can still maintain frequency/voltage (inverter generators are more flexible than synchronous types).
• Expected result: often a modest improvement from maintenance and correct mixture, with the biggest gains coming from operating point control rather than aggressive modifications.

Example 2: Getting more heat into a DIY water heater from a propane burner

• Goal: increase delivered heat to water without increasing fuel rate.
• Method: measure water flow and temperature rise; measure flue temperature.
• Changes to try: add baffles to increase hot gas contact time with the heat exchanger; insulate the combustion chamber where heat is not transferred to water; tune air/fuel to reduce excess air while maintaining clean combustion; reduce leaks that allow hot gases to bypass the exchanger.
• Expected result: lower flue temperature for the same water heating rate indicates improved heat transfer effectiveness (while still maintaining safe draft and avoiding condensation issues in inappropriate materials).

Example 3: Sustaining power in an air-cooled engine on a kart

• Goal: avoid power fade after 10 minutes at wide-open throttle.
• Method: log head temperature (or oil temperature) and note when power drops.
• Changes to try: restore factory shrouds and ducting; ensure fan is intact; slightly enrich mixture at high load; check timing is not over-advanced; improve airflow around the engine by shielding it from hot exhaust recirculation.
• Expected result: stable temperatures and consistent power, even if peak power is unchanged.