Shop Class: Forced Induction Fundamentals
Bring On The Boost
Pumping (compressing) ambient air into an engine’s intake manifold, adding to pressure created by the internal compression ratio, generates additional power from combustion. The concept goes back more than 100 years, with increasing relevance in current automotive production.
Mainstream naturally aspirated engines run solely on pressure created internally to provide combustion of the air/fuel mixture. The amount of pressure is determined by the volume of space within a cylinder and combustion chamber with the piston all the way down (bottom dead center), compared to the amount remaining after the piston travels all the way up (top dead center). This is the compression rati: 12 1/2-to-l means the volume at BDC is 12 1/2 times greater than that at TDC. The higher the ratio, the greater the air is compressed, and therefore higher pressure in the combustion chamber.
High compression has always been a high-performance mainstay, but with it comes a few hitches. When air is compressed to build higher pressures, temperature increases as well. Within a gasoline-burning four-stroke engine, if pressure/temperature applied to the air/fuel mixture is too high, it will ignite the mixture prior to the firing of the spark plug (preignition). In simple terms, this creates two separate explosions, which bang together and produce a knock (detonation, spark knock, ping). Along with the knock from abnormal combustion, extreme combustion temperatures are created, which can cause internal engine damage—literally burning a hole through a piston if conditions are right.
Knocking has always been an issue with gasoline internal combustion engines, but it’s held in check with the use of dynamic adjustments to air/fuel ratio, ignition timing, and exhaust gas recirculation (EGR), with the aid of a knock sensor input to the powertrain control module. However, when dealing with performance oriented high-compression, higher octane fuel is often a necessity. The higher the octane rating, the higher the temperature required to ignite the air/fuel mixture. Within a high compression engine, this prevents the unwanted ignition prior to the spark at the plug.
In 1885, German engineer Gottlieb Daimler patented the use of a gear-driven pump to force air into an internal combustion engine. This mechanically driven device went on to be known as a supercharger.
The turbocharger, compressor driven by the flow of exhaust gasses, was attributed to Alfred Büchi, a Swiss engineer researching diesel engine technology in 1905.
The amount of pressure inside the intake manifold above atmospheric pressure, achieved through either method, is accurately referred to as boost.
Pros and Cons
The mechanically driven supercharger has the advantage of immediate response in performance from boost pressure. However, at the same time, the load on the engine required to rotate the supercharger subtracts significantly from engine power (a loss usually compensated for by the pressurized induction).
A turbo has the opposite effect: less power loss by driving the compressor with exhaust gas flow but with a delay in boost, which diminishes low-end throttle response (turbo lag).
Typically, superchargers are belt-driven from the crankshaft. Modern automotive superchargers are used almost exclusively for high-performance applications, not efficiency. Turbocharging, not supercharging, is more suitable for mass-production cars concerned with fuel economy.
Rotating a supercharger may absorb as much as a third of the engine’s horsepower at the crankshaft. However, the direct boost overshadows the power loss as seen with response and total power output. That’s why we see 500ci supercharged engines in Top Fuel drag racing. Engines like these are not so worried about fuel economy, only power, peaking near 10,000 hp and consuming 4 to 5 gallons of nitro methane during a 1/4-mile run.
Roots-type blowers are where it all started with the use of counter-rotating shafts with an equal number of lobes (or paddles) intertwined to pump air. They don’t compress air internally but simply move it through the supercharger and into the intake manifold where the boost pressure forms. The amount of boost is regulated strictly by the rpm of the engine/supercharger, which establishes high amounts of pressure even at low rpm. Roots supercharges are a simple design and highly reliable, used in OE applications by GM, Ford, Mercedes, and Toyota.
Remember that increased pressure of the air/fuel mixture increases temperature consequently? That’s one drawback to the Roots-type supercharger. There’s no efficient method of cooling (intercooling) the boosted air.
Similar to a Roots-type supercharger, the screw-type uses two shafts with a screw thread design. Unlike the Roots-type, the variable pitch of the screws and tight clearances actually compress the air within the supercharger. This provides greater efficiency at high rpm and lower temperature increase of the compressed air.
A bit different, but efficient in its own right, a centrifugal supercharger uses an impeller to compress the air—similar to the compressor wheel of a turbocharger. While limited in the amount of pressure produced compared to a Roots or screw-type blower, centrifugal superchargers are somewhat smaller, produce less heat from compression, and easily network with a heat exchanger (intercooler).
A turbo is generally a centrifugal design with the use of an impeller to compress air just like its supercharger sibling. But the impeller (compressor wheel) is joined by a shaft to a turbine, as opposed to a drive belt. The turbine is kind of the opposite of an impeller. It rotates with exhaust flow and drives the impeller via the captured kinetic energy.
Again, disadvantages of turbocharging include the lag in boost and the high temperatures from the use of exhaust gasses, but unlike superchargers, boost can be dynamically controlled. This control has long been obtained with the use of a waste gate, which is basically a valve that varies the amount of exhaust routed to the turbine in conjunction with engine load. However, methods of control are becoming more advanced, as you’ll see with the different types of turbos.
This is the basis of the bunch: one turbine and one impeller inside the housing of one turbocharger, driven by the one main exhaust flow from the engine. It’s a simple and comparatively inexpensive design.
Two turbos on one engine can be used in parallel (half the exhaust through turbo #1 and half through turbo #2) or sequentially (exhaust moves through turbo #1, then turbo #2).
Two-stage twin turbos combine a large and small unit routed in series with the exhaust. Smaller turbos provide better power at low speeds (less turbo lag), while the larger is more efficient at higher speeds. The two can be controlled accordingly depending upon driving conditions, making the system more efficient.
Pay attention—this is a very cool setup. Twin-scroll means a turbo using two turbines (or one turbine wheel with two sets of blades) with separate inlet and outlet exhaust passages, fixed to a shaft driving the impeller.
With the use of a custom exhaust manifold, specific cylinder exhaust ports are routed to one scroll and the remaining cylinders to the other. Example: A four-cylinder engine (firing order 1-3-4-2) will have the exhaust from cylinders #1 and #4 routed to one scroll and that from cylinders #2 and #3 to the other.
The exhaust pulses from each cylinder come out the exhaust valves in the firing order sequence. Due to what we call valve overlap, half of the pulses are not in sync and interfere with each other. By separating the pulses, the conflict is eliminated and we produce a more efficient flow of exhaust to each of the two scrolls.
The turbine wheel and impeller therefore spin faster as compared to the same engine with a single-scroll turbo. This enhances boost as a whole and decreases turbo lag.
Variable Geometry (VGT)
This system uses variable vanes on the turbine as opposed to fixed blades. This allows for adjustment of the surface area contacted by exhaust, ultimately changing turbine and impeller speed in accordance with load.
VGTs are efficient, expensive, and used mostly on diesel applications due to the lower exhaust temperature. Gasoline exhaust temp is much higher and would require more exotic materials for the internal hardware to hold up.
Turbo lag due to limited exhaust flow at low rpm? How about using an electric motor to eliminate the issue altogether? This could possibly be the future of OE turbocharger production.
An electric motor can be used in tandem with exhaust flow spinning the turbine. Just like regenerative braking on a hybrid, the electric turbo motor can also be used as a generator. On deceleration when boost is unnecessary, the exhaust can continue spinning the turbine/generator and charge a high-voltage battery.
We don’t want hot intake air, and the majority of turbochargers use an intercooler between the turbo and the intake manifold. Just like a radiator, the intercooler removes part of the heat generated when the turbo compresses incoming air by using moving air or recirculating fluid.
Why So Many Turbodiesels?
Let’s get back to preignition. A gasoline-burning engine compresses the air/fuel mixture. If pressure and temperature are too high, the mixture may ignite prior to the spark plug doing its job. A diesel engine compresses only air, and the fuel is injected after the high pressure is formed nearing top dead center. The fuel ignites on contact with the high temperature air created from compression, so preignition from excessive pressure isn’t an issue with diesel applications.
This, along with a shorter power-range that limits turbo lag, makes diesel engines ideal candidates for turbocharging.
Up In The Air
Why is turbocharging so important in aviation?
Air density is the issue at high altitudes. The density decreases the higher we get, with less oxygen per cubic inch of air. So a naturally aspirated piston engine can eventually be starved for oxygen at a certain height.
A turbocharger’s boost pressure increases air density, giving us more oxygen in the combustion chamber to be burned along with additional fuel applied. Turbo boost can keep an aircraft engine running virtually the same as at sea level, even while flying at high altitudes.
The same air density principles apply with forced induction on the ground. Only down here we’re focusing more on power and efficiency, instead of maintaining flight.