Forced induction – it’s about numbers. Numbers matter. Ask your accountant. Ask your customer. Ask an engine builder. One number an engine builder loves is the one for an engine’s volumetric efficiency, or VE.
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VE is a measurement of how full an engine’s cylinder is with air. The number assumes that the air contained in the cylinder is at ambient pressure at sea level (14.7 psi). Obviously with an engine at rest each cylinder is 100 percent volumetric efficient, but we don’t care about engines at rest. The trick is achieving high VE when it matters most – under acceleration and at higher rpms. Under typical high rpm conditions air is moving into a cylinder courtesy of a piston traveling close to 50 feet per second, and that’s where the difficulty begins. Forced induction from superchargers and turbochargers lend a hand to increase engine VE.
More Air = More Power
Every tech knows more air means more power. That’s always been the rule with engine performance whether you’re grading performance on power or efficiency. Not that fuel and spark deserve less than honorable mentions. Air volume and density is the biggest factor when you want to go fast or go efficient or in the realm of today’s automotive market – plenty of both.
2012 Chevy Cruse with 1.4 Turbo. Green is throttle, blue is RPMs, red is turbo bypass and magenta is turbo wastegate. When scanning make sure boost goes up with load and throttle increases and down with decel.
Consider that the average catalytic converter equipped non-direct injection spark ignited engine has an optimal stoichiometric air:fuel (AF) ratio of 14.7 pounds of air to every 1 pound of fuel. Take into consideration the vast difference between an actual volume of 1 pound of fuel and 14.7 pounds of air. One pound of fuel would fill a 20 ounce empty plastic soda bottle from a vending machine. Now in order to really calculate how much air 14.7 pounds is (volume wise) we could do a bunch of complex math or just look it up for an assumed pressure of sea level and temperature of 70 degrees F. Those numbers considered, 14.7 pounds of air is 2,894 gallons.
That’s right – for every one 20-ounce soda bottle of gasoline an engine consumes it breathes enough air to fill a small gasoline delivery truck’s tank. Now that everyone’s head is around the mental picture of a soda bottle of fuel for every tanker truck tank of air we can really appreciate the importance of volumetric efficiency and the role of a blower – a supercharger or turbocharger.
While VE can be improved with induction and exhaust manifold deigns, valve timing and a host of other design tricks, nothing beats the blower! A turbine drawing in ambient pressure air and compressing it to two or more atmospheres (30 PSI or greater) increases the VE and thus performance of an engine in an impressive manner.
Blower 101
In a general sense, there is nothing new about either style of VE boosting blowers. Superchargers utilize an engine driven turbine that compresses air (thereby increasing air pressure and density) via a chain, gear or belt. The result is low-end torque along with a horsepower boost at higher rpms. Trucks, muscle cars, dragsters, etc. have been traditional users of the supercharger for many, many decades. While saddling the crankshaft with another mission (turning a supercharger) does drop fuel economy ever so slightly, the supercharger shines with instant power that will make a believer out of anyone behind the wheel.
Turbo chargers on the other hand, use exhaust gas driven blowers to increase high-end horsepower. Trucks, performance cars,and longer duration racing vehicles (Formula 1, CART, etc.) all are traditional candidates for turbocharging. For these reasons, blowers are quite at home on diesel-equipped trucks and some non-performance oriented passenger cars. You will be hard pressed to find a light duty (LD) diesel truck today without a turbocharger.
Late 90’s GM 3800 V6 Supercharger. Small throttle plate looking bypass for boost opens when ECU controlled vacuum solenoid activates.
Although turbo and superchargers have been around for over 125 years, the turbo has been an exception to the rule on most domestic passenger cars and gas engine equipped LD trucks for most of those years. In recent years, trends resulting from stricter emission and environmental standards have resulted in the turbo’s exposure into the economy car market. Take for example the Chevrolet Cruze 1.4 liter 4 cylinder engine. The Cruze isn’t exactly a mini car nor is it stripped down to be ultra-light as are some imported economy cars so 1.4 liters under the hood hardly sounds like enough power until its turbocharger is factored in. Performance is quite adequate while mileage nears the numbers that are typically only met with a hybrid electric vehicle.
Not to be outdone, Ford’s approach to this increasingly popular technology is marketed as “EcoBoost” and as is with the case of some of the GM applications, Ford utilizes a smaller engine with a turbocharger to do “more with less.” Ford pairs their gasoline direct injection (GDI) with their turbo to reduce the delay inherent with most turbos commonly referred to as “turbo lag.” The combination increases fuel economy by 20 percent, reduces emissions by 15 percent and still provides a noticeable improvement in power.
The Hardware (and Software)
Hardware for the supercharger varies with the design of the blower internally. Rotor superchargers, centrifugal superchargers and vane superchargers comprise the most popular variety with the passenger car market. The physically longer design used on vehicles like the Buick 3800 and on street rods/top fuel dragsters use a set of two rotors. Buick, Pontiac and Oldsmobile 3800 V6 models were popular cars to sport the option of the Eaton M62 and M90 blowers.
These units are often called Roots after the company/inventor name. Centrifugal superchargers are compact units utilizing a spinning turbine. These typically are used in tight spaces such as bolt on aftermarket units although some late model Ford Mustangs, Thunderbirds and limited edition F-150s did come from the factory supercharged. The common GM Roots-style blower’s main components were the compressor housing, (part of the upper intake manifold on the GM V6 designs) the nose drive unit containing a drive pulley, coupler, bearings and a boost bypass shaft with associated bypass actuator and vacuum solenoid.
The job of the bypass shaft/plate is to allow normally aspirated air to enter the engine in a non-compressed state thereby eliminating or reducing boost. During periods of rapid decel or after engine management electronics have detected certain problems the boost bypass solenoid is activated to apply vacuum to the vacuum actuator. The actuator in turn moves a rod connected to the bypass shaft. The bypass shaft has a butterfly plate on it resembling a small throttle plate.
Hardware for the typical turbocharger contains the turbine wheel, (exhaust driven fan) turbine housing, turbo shaft bearing housing, compressor wheel, (compresses air) compressor housing, waste gate and waste gate actuator.
The wastegate is a valve that allows exhaust gas flow to bypass the exhaust driven turbo when conditions merit that boost be limited as in the case with the supercharger boost bypass valve. Wastegates can be mechanically controlled by the mechanical dynamics of boost pressure versus vacuum against a spring loaded diaphragm. The diaphragm in turn moves a rod, which is connected to the wastegate valve itself. The rod typically has traditionally had the appearance of being adjustable and is – by the factory only in most cases.
Now that I’ve made that disclaimer, some techs will have to fix the DIY or other “tech” who worked on it to adjust the boost up to (or down to) specs after the rod has been adjusted. Removal from the turbo is required in order to shorten (increase boost) or lengthen the rod (decrease boost). More recent turbos, like those on some modern common rail diesels, the wastegate actuator control is part mechanical (spring loaded diaphragm versus boost pressure / exhaust back pressure) and part electronic vacuum solenoid control courtesy of the engine management computer.
To add to further complexity (not that any OEM engineer ever does so on purpose) another advance in recent model years for turbo chargers is the variable geometry (displacement) turbocharger. They work off of the Bernoulli principle. A ring surrounds the exhaust turbine that contains a set of nine or so vanes shaped like an airplane wing. As the ring is rotated by an actuator, the position of the vanes change and like the flaps on an airplane wing, pressure is changed. This changes the turbine’s speed (and engine boost) and along with that, exhaust backpressure.
Late model common rail diesel variable geometry turbo with actuator / control ECU on side.
Increasing exhaust back pressure is desirable for a diesel to assist with much needed EGR. Typical to newer diesels the variable geometry turbo actuator is moved hydraulically via electronic controls. The actuator and associated controls are in the center of the turbo housing and include an actuator position sensor and a turbine shaft speed sensor. The actuator electronic control may be a full-blown ECU on the CAN communications bus. In these cases, the engine coolant may be used to keep the ECU alive in that hot environment.
Not wanting to be outdone by the diesel engineers, GM’s gas engine powertrain engineers introduced a boosted air bypass valve into the cold air side of their small engine. The 1.4 liter Chevrolet Cruse is an example. Here is how it works. When the accelerator pedal is depressed this bypass valve is closed. The force in the return spring integrated in the valve presses the valve cone against its seat in the turbo housing. The valve is turned off when the accelerator pedal is released,
In order to avoid pressure spikes in the intake manifold and unloading or overrunning the turbo, the engine control module (ECM) sends a voltage signal to the bypass valve, which will then open. The compressed air on the pressure side of the turbo is led to the intake via the open valve. When the pressure drops, the turbine speed can be kept relatively high and the turbocharger is prevented from exceeding the pump limit. This is all done on the cold side (intake) of the system. There is still a wastegate for the vehicle’s turbocharger.
Speaking of the cold side, last but not least in the world of hardware would be the heat exchanger referred to as an intercooler. You don’t have to design in an intercooler to build a vehicle with a blower but it sure helps. Back to the ideal gas law – if you compress a gas the temperature goes up with the pressure. We don’t need to add more heat into a NOx/Knock prone environment so we cool the air prior to the intake manifold. Intercoolers may be mounted in front of the radiator/condenser or on top of the engine.
They may be air-cooled or water-cooled. They are important. When they get restricted for airflow (internally or externally) the very technology that was used to increase VE is now decreasing it.
Oil Can, Please
Superchargers spinning at 50,000 rpms and their counterpart turbo spinning at 100,000 rpms require a sufficient amount of uncontaminated lubrication. Although not restricted to a stationary oil sump design, the Eaton Roots-style blower used in GM’s passenger car line applications utilized a sump with its own oil.
Regardless how neglected the motor oil got in the crankcase, the supercharger still has a fighting chance. The oil should be changed whenever bearing failure has occurred or any other source of contamination or abusive service (racing, towing, etc.) is suspected. In the case of the popular GM V6 applications using Eaton blowers, the service procedure requires two bottles of Supercharger Oil (P/N 10953513) unless you opt to not flush the nose drive assembly.
You don’t necessarily have to remove the unit from the engine to drain the oil once the ¼-inch drain plug is removed. Simply use a floor jack to raise the driver’s side of the vehicle to push the oil towards the front of the unit. Using two large syringes (farm supply store) and some ¼-inch clear tubing, you can then suction out the oil from the unit. Lower the floor jack to bring the vehicle level again and use the syringes and clear tubing to add oil until it reaches just below the bottom of the threads of the drain hole. Using clear tubing will help you see when the oil reaches the bottom of the threads.
Turbocharger lubrication is supplied in most all cases from the engine’s lubrication system, which leads to a bit of a dilemma these days. On one hand, we have the traditional 3,000-mile oil change for all but rough service (short trips or hot usage vehicles) and on the other hand we have longer life oils, better filters and the ever so popular oil life monitor that tells the customer when to get their oil changed. Oil life monitors are based on engine Parameter Identifiers (PIDs) like the vehicle speed sensor (VSS) for distance and the engine coolant temperature sensor (ECT) sensor for time at operating temperature to produce a software model that allows for some of the factors in oil breakdown but what about other factors like dusty roads and cheap air filters?
According to one OEM dealer service manager in my hometown, the hype about the waste of money and cost to the environment due to the myth of the 3,000-mile oil change has resulted in the opposite extreme: oil that doesn’t get changed often enough. Drivers are waiting the 7,800 to 20,000 miles of driving with their synthetic oil waiting for that oil life monitor to clock down to zero percent and according to my service manager friend’s experience with a rash of engine problems like premature valve guide wear that’s too late.
When we are talking about a hot, high-revving turbocharger a return to slightly more conservative oil change intervals is in order to prevent premature turbo wear out. Oil is cheap. Turbos are not.
Diagnosing Blower Problems
Noise is the big complaint with both turbochargers and superchargers due to wear out. Keep in mind some noise is normal. Some turbo or supercharger whine is the nature of the beast. If a pulley has been changed out on a GM supercharger (smaller one for more blower speed / boost) there will be more natural noise from the supercharger.
When noise exceeds the norm, it’s time to diagnose why. If you can get to either side of the turbine on a turbocharger (cold side will be easier) simply use your fingertip to check for play up and down. There should not be any noticeable bearing movement with upward and downward motion. When a shaft bearing is loose or worn out on a turbo, it often is the oil issue previously mentioned. Bearing noises are of course expected but on some of the diesels due to design and location worn out bearings can make a noise you might swear is an exhaust leak but actually is a worn turbocharger bearing.
With a high mileage older supercharger (i.e. a 1990s something Buick) it might just be wear out with something in the nose drive unit. Those noises come in two varieties: bearing wear and coupler wear. If the noise is grinding or whining the bearings are the likely sources. If there is a noise like something rattling or loose look for a bad coupler. The coupler’s job is to connect the pulley driven input shaft to the rotors internal to the supercharger.
As the engine’s drive belt fluctuates in speed, couplers dampen those variations to protect the supercharger from damage. A quick test to determine which issue the supercharger has is to remove the drive belt and rotate the pulley back and forth. If there is a clunking sound – it’s most likely a coupler problem.
Next, grab the pulley and check end play up and down and side to side. If there is excessive movement, bearings are the problem. Numerous resources are available on the aftermarket for everything from supercharger coupler kits to rebuilt turbochargers. When selecting a reman blower, consider the time tested adage “you get what you pay for.” Murphy’s Law also applies in these cases when you consider the money you saved on a cheap reman turbo or supercharger is directly relational to the complexity of the R&R when the job comes back.
Engine management failures to look for would be over boost (detonation / engine damage) and under boost. In the under boost situation, the symptom can be slight to a full blown “won’t hardly move down the road.” Almost everything is a PID these days, so if you have a decent scan tool look for things like turbine shaft speed errors or actuator position sensor diagnostic trouble codes (DTCs) that would indicate that something is physically broke (won’t move to the correct position) or the sensor has failed.
Intercooler built into top of Ford Powerstroke diesel engine
As with any new technology, the combination of the boost bypass and wastegate control on the GM economy engines will be a steep learning curve to determine on a road test that something is a little off. When they are vastly out of specs DTCs typically set but in the real world we often have to battle the no code drivability problems due to worn mechanical components or slightly skewed sensors. Start scanning those new engines with complex forced induction systems as they come in for routine service so you’ll have your sea legs when a real problem comes into the bay.
As progress marches on, we’ll be seeing series turbos become more popular. These systems compress, intercool, then compress and intercool some more with dual turbos that are designed to have one handle low-end rpm duties and the other handle the higher rpm duties. In the months and years to come as fuel prices continue to rise, we’ll be glad to see our fuel costs decrease as our motor’s VE numbers increase.
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