Under Pressure, Part 1June 6, 2018
The basics of turbocharging
If ever there were a mechanical marriage made in Heaven it is a diesel engine and a turbocharger. On the farm this union is found in everything from a pick-up truck to a combine.
There are two ways an engine can breathe: naturally via the differential in pressure in the cylinder bore versus the atmosphere (naturally aspirated) or by force through either a turbocharger or supercharger (forced induction). The two methods of forced induction differ in how they are powered. A supercharger is driven from the engine’s crankshaft and consumes power. A turbocharger uses the exhaust gas exiting the cylinder to operate it and takes no power from the engine.
The turbocharger accomplishes two things. It fills the cylinder bore with more air and by inducing turbulence (in the cylinder) it greatly improves combustion. A turbocharger makes a diesel more powerful, run cleaner, and have the potential to use less fuel.
The metric used to measure cylinder fill is called volumetric efficiency (VE) and is read in percent. A naturally aspirated engine experiences around 80% VE or in other words, uses 80% of its capacity in regard to cylinder volume. By employing forced induction, the VE can improve to 100% and higher based upon the amount of air flow created and the pressure produced. The pressure is read in the intake manifold as boost.
The dashboard gauge reads psi but it is really the amount of pressure over the atmosphere. If atmospheric pressure is 14.7 psi and the turbocharger is producing 14.7 psi (dash gauge reading) then the cylinder is actually seeing 29.4 psi. Thus, the effective size of the engine can be considered doubled for every 14.7 psi of boost. In theory, a 12-liter engine (1 liter is approximately 61 cubic inches) when exposed to around 15 psi of boost pressure is the equivalent of a 24-liter engine that breathes naturally.
What is wonderful about turbocharging is that the power gain is passive —– it is only there when you need it. When the load is low the engine is its mechanical size but when the need is there it responds as a much larger one.
Many applications also employ a heat exchanger identified as either an intercooler or charge air cooler (CAC). Its purpose is to cool and in turn increase the density of the air entering the cylinders. The action of the turbocharger heats the air which is undesirable —- it contains less oxygen molecules. For every 10 degrees F change in charge air temperature, power is altered by one percent. Cooler air makes more power and hotter air less.
Hot and cold
A turbocharger incorporates an exhaust driven turbine wheel that is contained in a volute (snail-shaped housing). It is connected via a shaft to another volute that has a centrifugal compressor wheel that sends the charge air to the intake manifold. The turbine side is considered “hot” and the compressor side “cold”.
The shaft rides in a housing that is fed engine oil under pressure with a drain back to usually the oil pan or timing cover. The bearings are of the floating variety. Some applications and especially earlier designs may have semi-floating or pressed-in ball or roller bearings.
Under high boost conditions the shaft and thus the turbine and compressor wheel can spin at speeds as much as 150,000 rpm.
Due to the exhaust heat at the turbine, many models also send engine coolant through the bearing housing to improve service life and reduce coking of the oil.
Seals are used to keep the oil away from the exhaust and intake tracks along with keeping the spent gasses and boost pressure in the volute.
A means to control boost pressure is employed and can either be a wastegate, movable ring with vanes, or an inherent clearance on the turbine side. A wastegate allows exhaust gas to bypass the turbine wheel and housing and thus, limits its speed.
Function of load not engine speed
The energy that turns the turbine wheel is from the hot exhaust gas leaving the cylinder. The turbocharger is passive since it responds to a much lesser extent to engine crankshaft speed than exhaust temperature.
That is why you will hear the turbo spin up when the engine is loaded even though there may be little to no increase in engine speed.
As the load on the engine increases so do the exhaust temperature and its velocity. When the exhaust leaves the port of the cylinder head the inert gas experiences isentropic expansion. This means without temperature change. The hot and expanding gases are forced into the turbine housing and acts on the turbine wheel in the same manner an old grist mill would have the flow of a river power it. The compressor wheel then feeds air to the intake manifold under pressure. The result is an increase in VE, power and reduced emissions.
Also see: Part 2: Turbocharger service tips