Turbo Definitions

[West]

This is a reference of common turbocharger definitions.

ABSOLUTE PRESSURE - This term refers to pressure measured on the scale that has its zero point at approximately 14.7 psi (at sea level) below atmospheric pressure. It is a true measurement of all the pressure, rather than just the pressure above atmospheric. See Gauge pressure.

ABSOLUTE TEMPERATURE - Similar to absolute pressure, absolute temperature has its zero point where no heat exists. This is approximately 460°F below 0°F. An absolute degree is the same size as a Fahrenheit degree. The freezing point of water (32°F) is about 492°F above absolute zero, or 492° absolute.

AIR/FUEL RATIO {AFR) - AFR is the ratio of the weight of air to the weight of fuel in a combustible mixture. AFR is critical in the proper functioning of an engine.

AMBIENT - Ambient refers to the surrounding atmospheric pressure and temperature.

ATMOSPHERIC - This word has recently taken on the connotation of an engine operating without any form of supercharger. My lawn mower has an atmospheric engine.

BLOW-THROUGH - This indicates that the throttle is on the outlet side of the turbo compressor. See DRAW-THROUGH.

BOOST - Boost is pressure above atmospheric, measured in the intake manifold.

BOOST THRESHOLD or BOOST POINT - This is the lowest engine rpm at which boost from the turbocharger will increase power over the engine's atmospheric equivalent. More simply, the lowest rpm at which noticeable boost (usually 1-2 psi) can be achieved.

BYPASS VALVE - The bypass valve permits a bleed of flow around the turbo when the engine is not under boost.

CLEARANCE VOLUME - Combustion chamber volume above the piston at top dead center is called clearance volume.

COMPRESSION RATIO - This is displacement volume plus clearance volume divided by clearance volume.

COMPRESSOR - In this book, the compressor is the air pump itself-the front half of the turbo, through which intake air passes. It is also frequently referred to as the "cold" side.

COMPRESSOR EFFICIENCY {Ec} - Efficiency is the ratio of what really happens to what should happen. In the case of the compressor, measurement of the temperature gain caused by compressing the air exceeds what thermodynamics says it should be. Compressor efficiency converts calculated temperature gains to real temperature gains.

COMPRESSOR SURGE - Compressor surge occurs when the throttle is slammed shut and air is caught between a pumping turbo and the throttle plate. This air blasts its way backward out the front of the turbo. When this happens, there is suddenly room for more air in the manifold, and air is pumped back in by the still-spinning turbo. The throttle is still closed, so the air again blasts back out through the turbo. This continues until the turbo loses enough speed for leak back around the compressor to dampen the air oscillations. Compressor surge can also occur under boost if too much boost pressure is present with low airflow through the system. The chirping sound heard from the turbo when lifting off the throttle while operating under boost results from this oscillating air volume. This noise is suppressed by the bypass valve.

CROSSOVER POINT - This is the point at which manifold boost pressure equals turbine inlet pressure.

DETONATION - Detonation is spontaneous combustion of the air/fuel mixture ahead of the flame front. When pressure and temperature exceed that required for controlled combustion, the mixture auto ignites. The metallic pinging sound is the resulting explosion's shock wave colliding with the cylinder walls.
Note: Ping, knock, and detonation are equivalent terms. Pre-ignition is an altogether different beastie. Do not call one the other.

DISPLACEMENT VOLUME - Displacement volume may be defined in several ways: (1) the swept volume of the cylinder; (2) the area of the bore times the length of the stroke; (3) total engine displacement divided by the number of cylinders.

DRAW-THROUGH - This indicates that the throttle is on the inlet side of the turbo compressor. See BLOW-THROUGH.

END GAS - The end gas is the last part of the air/fuel mixture to burn. Its importance to a turbocharged engine is paramount, because it is this end gas in which detonation usually occurs.

GAUGE PRESSURE - Gauge pressure is the scale that reads zero at atmospheric pressure. All references to boost pressure in this book will refer to gauge pressure. For example, 5 psi boost would be 5 psi above atmospheric pressure.

IN. HG - This phrase reads "inches of mercury" and is a measure of pressure on yet a different scale. In this book, in. hg will refer to vacuum in the intake manifold, and the scale works downward toward atmospheric pressure. For example, idle speed vacuum is usually about 18 in. hg, and as throttle is applied, the vacuum goes toward O gauge, which is atmospheric pressure.

INTERCOOLER - An intercooler is a heat exchanger placed between the turbo and engine to remove heat from air exiting the turbo when operating under boost. Intercoolers are also called charge air coolers.

INTERCOOLER EFFICIENCY (E.> - An intercooler's efficiency is measured by how much heat it removes relative to the heat added by the compressor.

INERTIAL LOAD - Inertial loads are those created by weight and acceleration. A heavier piston creates a greater inertial load. Likewise, an increase in rpm means greater acceleration and, thus, a greater inertial load.

LAG - Lag is the delay between a change in throttle and the production of noticeable boost when engine rpm is in a range in which boost can be achieved.

LEAN - Lean means not enough fuel to achieve the correct air/fuel ratio for the existing conditions.

NONSEQUENTIAL FUEL INJECTION - EFI that pulses independently of the intake valve position is non sequential.

GEM - Original Equipment Manufacturer; the company that built it in the first place.

POWER - Strictly speaking, power is the result of how fast a certain amount of work is done. In the automotive context, power is the product of torque at any specific rpm times that rpm.

POWER LOAD - This is the load induced into all engine components by pressure created by the burning gases.

PRE-DETONATION - This is a meaningless phrase and should not be included here or anywhere else.

PRE-IGNITION - Pre-ignition refers to spontaneous combustion of the air/fuel mixture prior to the spark.

PRESSURE RATIO - The ratio of absolute boost pressure to atmospheric pressure is called the pressure ratio.

PULSE DURATION - The amount of time, measured in thousandths ofa second (msec), that an electronic fuel injector is held open on any single pulse. Pulse duration is a relative measurement of the amount of fuel delivered to one cylinder per combustion cycle.

REVERSION - Reversion occurs when some of the burned exhaust gases are pushed back into the combustion chamber and intake system during valve overlap. This is caused by exhaust manifold pressure exceeding intake pressure or by shock waves in the exhaust ports and manifolds.

RICH. Rich is the condition that exists when too much fuel is present to achieve a maximum power air/fuel ratio.

SEQUENTIAL FUEL INJECTION - A fuel injector pulse timed to discharge fuel when the intake valve is in the most advantageous position is called sequenced. It pulses the injectors in the same sequence as the firing order.

SUPERCHARGE - To force more air into an engine than the engine can breathe by it self is to supercharge it. A supercharger is the device that does this. It may be driven by a belt, gears, or a turbine. When driven by a turbine, it is called a turbocharger.

THERMAL EFFICIENCY - See COMPRESSOR EFFICIENCY, INTERCOOLER EFFICIENCY.

THERMAL LOAD - In this book, thermal load will take the rather narrow definition of heat added to the system by the turbocharger. This comes from heat produced in the air that is compressed by the turbo and the mixture heat increase due to reversion.

THROTTLE RESPONSE - A change in the speed or torque of an engine brought about by a change in throttle position is called throttle response. Throttle response should not be confused with turbo response.

TORQUE - The amount of twisting force provided by a turning shaft is called torque. It is measured in foot-pounds, inch-pounds, or Newton meters.

TURBINE - The turbine is the fan driven by the engine's exhaust gases. It is often called the "hot" side of the turbocharger.

TURBOCHARGER - A turbocharger is a supercharger driven by a turbine.

UNDER BOOST - When a system has greater-than-atmospheric pressure in the intake manifold, it is operating under boost.

VOLUMETRIC EFFICIENCY (Ey) - This is the ratio of the number of molecules of air that actually get into a combustion chamber to the number of molecules in an equal volume at atmospheric pressure. For atmospheric engines, this ratio is almost always less than one. Supercharged engines are capable of operating at ratios greater than one.

WASTEGATE - The wastegate is a boost-pressure-actuated valve that allows only enough exhaust gas into the turbine to achieve desired boost. The wastegate routes the remainder of the exhaust gas around the turbine and out the tailpipe.

[sharonk868]

Thanks you for the post.
Hi guys, Im a newbie. Nice to join this forum.

[Future240]

Wonder if I should rip this and put it on my thread......

[peterson]

Im a newbie. Nice to join this forum.

[Future240]

I have no idea what that means, but thanks, welcome to the site and keep posting info.

[Future240]

Thanks for the info

[anthony]

it was nice to speak Inertial loads are those created by weight and acceleration. A heavier piston creates a greater inertial load. Likewise, an increase in rpm means greater acceleration and, thus, a greater inertial load.

[Future240]

What do you mean by rather narrow definition?

[roger]

the lowest engine rpm at which boost from the turbocharger will increase power over the engine's atmospheric equivalent. More simply, the lowest rpm at which noticeable boost (usually 1-2 psi) can be achieved.

[Rooney]

A fuel injector pulse timed to discharge fuel when the intake valve is in the most advantageous position is called sequenced. It pulses the injectors in the same sequence as the firing order.

[franklin]

In this book, the compressor is the air pump itself-the front half of the turbo, through which intake air passes. It is also frequently referred to as the "cold" side.