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Increased air flow boosts engine power by Kim Barnett, KONDININ GROUP he easiest and most cost-efficient way of increasing engine power is through forced air induction either by supercharging, turbocharging or a combination of both. Superchargers are mechanically driven, usually through a crankshaft mounted pulley belt drive or gear train system. Directly driving the supercharger tends to produce a more constant ‘boost pressure’ and air flow rate across the engine’s power range as supercharger impeller speed is directly linked to engine speed. Turbochargers are driven by exhaust gas flow from the engine which is essentially waste energy. Peak turbocharger efficiency occurs when exhaust gas flow is high so boost pressure at low engine speed can sometimes result in minor performance limitations. But both systems have limitations. Due to their direct drive arrangement superchargers are less able to compensate for variations such as reduced air pressure at high altitudes or air flow restrictions caused by air filter blockages. Turbochargers are overdriven in their operating range with excess drive energy from exhaust gas flow bypassing the turbocharger by ‘waste-gate’ control. This excess drive energy allows compressor power to be increased to help compensate for low inlet air pressure or partial restrictions to inlet air flow. Superchargers require engine power to be driven which places additional mechanical loads on the engine crankshaft. Turbochargers do not produce a mechanical load to the engine’s rotating components and use waste energy from the engine as the drive power. When set up correctly a turbocharger can improve engine efficiency by eliminating the need for muffler baffles as exhaust noise is reduced as it passes through the impeller. At peak efficiency the turbocharger will have a ‘scavenging effect’ which extracts exhaust gas from the combustion chamber further improving overall engine efficiency. Despite limitations, supercharging or turbocharging is still the most efficient and reliable way of improving engine torque and pulling power.
On-farm use Diesel engine manufacturers have moved away from supercharging in favour of turbocharging. Turbocharger components are generally cheaper and any shortcomings historically linked with turbocharging are not so critical for diesel engines. Turbocharging limitations such as delayed engine response (turbo lag) and reduced power at low engine speed have been largely eliminated FARMING AHEAD No. 82 - October 1998
Kim Barnett
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Whether superchargers or turbochargers are fitted as original equipment or an aftermarket modification, a fully integrated system is needed where engine basics such as fuel control, air filters, exhaust components and instrumentation are designed to operate as a package.
in the new generation of turbochargers and with effective waste gate control. For farm applications most diesel engines will operate at a constant speed at the higher end of the engine operating range. Supercharging has recently made a comeback as an efficient alternative with some more sophisticated engine designs using both supercharging and turbocharging to balance engine power and fuel consumption. There have also been significant design developments particularly with ‘screw’ type superchargers and ‘high helix’ designs for vane type superchargers. Unfortunately, the simplicity of this engine modification can lead to problems as many basic engine design considerations are overlooked when the turbocharger or supercharger installation is regarded as ‘bolton’ horsepower. Most modern engines are designed to burn as much fuel as possible for the amount of oxygen available in the combustion chamber. Their efficiency is determined by how
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GLANCE • The most effective method of improving fuel burning efficiency in an engine is by forced air induction from superchargers or turbochargers. • Increasing power output from an engine will result in more internal mechanical stress and heat generated within the engine. • Modifying a vehicle’s engine to produce more power or exceeding recommended towing loads can void the vehicle manufacturer’s warranty.
completely the fuel burns which is directly related to available oxygen. Increasing fuel supply to an engine without providing additional oxygen may reduce the power output and will generally degrade fuel consumption rates. Conventional internal combustion engines rely on air drawn from the atmosphere carrying oxygen into the combustion chamber to burn the fuel. Atmospheric air is about 20 per cent oxygen, making this method of oxygen delivery inefficient. But atmospheric air is the most abundant, cheapest and readily available source of oxygen particularly for farm machinery and motor vehicles.
More oxygen equals more power The only way to effectively increase combustion energy and therefore engine power is to provide more oxygen to enable more fuel to burn. This can be achieved by delivering more air into the engine cylinders or providing an extra source of oxygen. For example, some piston-engined military aircraft use compressed nitrous oxide gas fed directly into the engine to increase oxygen supply. The gas is injected when extra short-term power is required or to compensate for low oxygen levels at high altitudes. Naturally-aspirated engines rely on air being drawn into the cylinder during the intake stroke of the piston. Enlarging the engine cylinder size by using a larger piston diameter or lengthened piston stroke can increase air volume for fuel combustion. But increasing engine size to produce extra power has its drawbacks such as increased weight and size and more mechanical losses due to the extra weight of internal reciprocating components. An alternative is to force more air into a smaller cylinder using an air compressor or pump. When the inlet valve on a normallyaspirated engine opens atmospheric pressure 29
forces air inside the cylinder. Engines often produce less power at high altitudes due to the lower atmospheric pressure reducing the volume of air entering the cylinder. The function of a supercharger or turbocharger is to increase the air pressure applied to the inlet of an engine resulting in a greater volume of air being forced into the cylinder.
Experienced operators use the exhaust temperature gauge to establish correct gear and load settings and to optimise fuel consumption.
Warranty and service
Compression When an inlet valve is closed at the bottom of the stroke, air and fuel caught in the cylinder is compressed by the stroke of the piston. Compression ratio is the engine specification which rates how much the airfuel mix is compressed. For example, petrol engines using unleaded fuel can have a compression ratio of 7.5 to one. This means the internal volume when the piston is at the bottom of the cylinder is 7.5 times more than the volume of the combustion chamber when the piston is at the top of its stroke. In supercharged or turbocharged engines the pressure inside the cylinder at the start of the compression stroke is increased. When this increased pressure is magnified by the same compression ratio the final pressure inside the combustion chamber is significantly increased. But there are limits to combustion chamber pressure. Achieving maximum combustion chamber pressure without causing ‘premature detonation’ is a delicate balancing act. Premature detonation is often referred to as ‘pre-ignition’ or ‘self-detonation’ and occurs when the compressed fuel-air mixture explodes before it is supposed to. In a petrol engine this will be before the spark plug fires or in a diesel engine it will be before the fuelair mixture is designed to self-detonate. Premature detonation reduces engine power and causes permanent engine damage due to pistons and other internal components being exposed to excessive shock loads. Some more sophisticated engines are fitted with electronic ‘knock’ sensors which detect premature detonation. This signals the engine’s control systems to adjust ignition and fuel feed. Overheating, excessive engine load, incorrect fuel-air mixture or incorrect ignition or diesel fuel pulse timing can contribute to premature detonation. Supercharging or turbocharging an engine increases combustion chamber pressure and can cause premature detonation unless the engine is adjusted to compensate for the increased pressure. Petrol engines intended to operate with high ‘boost’ pressure may be modified to increase combustion chamber volume by fitting a decompression plate between the cylinder and cylinder head or by fitting pistons with a recessed crown.
Engine temperature Increased heat energy generated by higher 30
Increasing combustion chamber pressure by forced air induction in diesel engines requires the recalibration of fuel control components such as pumps and injectors to perform efficiently.
combustion rates inside a supercharged or turbocharged engine must be removed from the combustion chamber area through the engine’s cooling system. Engines vary in their capacity to carry away additional heat and this is primarily a function of cylinder head design, coolant type and the condition of cooling cores within the cylinder head. Heat from the engine is released through the radiator. Recommendations are to fit a heavy duty radiator and locked radiator fan to vehicles fitted with superchargers or turbochargers particularly when towing. Under continuous heavy load conditions an external engine oil cooler may be required to minimise internal heat build-up and to maintain oil at its optimum operating temperature. Seek qualified advice to determine if a particular engine can cope with the extra heat generated by supercharging or turbocharging.
Exhaust temperature Vital to a supercharged or turbocharged engine is an exhaust temperature gauge. The temperature under load will always be high for this type of engine due to its increased combustion rate. The gauge can be monitored for signs of engine performance, fuel use, detecting engine failure and overheating. Under heavy loads the combustion chamber temperature can build up to where pistons and valves can be damaged. This overheating does not register immediately on the water temperature gauge so damage to internal components may have already occurred. Exhaust temperature sensors are installed close to the exhaust manifold and will quickly register temperature variations. Excessive temperatures can also indicate an overloaded engine. When over-fuelling occurs particularly in diesel engines any unburnt fuel is pumped into the exhaust system where it continues to burn and registers as increased exhaust temperature. By selecting a lower gear or reducing the throttle setting, all fuel burns inside the combustion chamber resulting in more efficient engine operation and lower exhaust temperature.
If a turbocharged or supercharged engine is required the safest course is to select an original equipment manufactured (OEM) unit especially in terms of warranty and service. The unit is installed by the vehicle manufacturer or at an authorised service workshop using parts supplied and certified by the manufacturer. By using an OEM installation there is only one stop for warranty claims. Warranty claims for some after market installations can be frustrated by uncertainty and arguments over who is responsible for the claim — the vehicle manufacturer, supercharger or turbocharger equipment supplier or after market installer. With an OEM installation manufacturers are responsible for designing the vehicle’s transmission and drivetrain to withstand the extra torque produced by the engine and to ensure the engine’s efficiency and reliability. Supercharger or turbocharger installations which are OEM authorised have generally been developed through extensive testing. After market installations can vary widely in cost, quality, warranty and service support. There are many highly effective aftermarket installations worth considering because: • High quality, thoroughly developed supercharger and turbocharger systems are available as after market installations at a significantly reduced cost compared with OEM-fitted systems. • OEM systems are often based on conservative boost pressures resulting in only moderate gains in engine torque and power output. A good quality fully integrated after market system may produce a more substantial power gain with reliable long-term engine life. • Cost of OEM parts may be prohibitive or the parts may be unavailable, particularly when fitting to an existing vehicle. • Aftermarket suppliers have varying levels of service and warranty agreements with vehicle manufacturers. At least one company is pursuing manufacturer endorsement of an underwritten warranty insurance system.
FARMING AHEAD No. 82 - October 1998
Overloading Warranties can be voided if the vehicle is overloaded or unauthorised adjustments are made to critical components, regardless of whether it is an OEM or aftermarket installation. Some light four-wheel-drive vehicles are being fitted with superchargers or turbochargers to produce extra power for heavy on-farm applications such as boomspraying. Most 4WD vehicles have a maximum towing capacity of about 2800 kilograms. A moderately sized boomspray with full tanks weighs more than 5000kg. Overloading of this magnitude combined with low gear towing over continuous rough terrain places severe stress loads on the engine, chassis and transmission and falls outside recommended loads and driving conditions for vehicle warranties. There is evidence also that some owners are adjusting the calibrated diesel fuel pumps fitted to their vehicles to increase engine power. Adjustments often involve breaking the lead calibration seal automatically voiding warranty.
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