Developments in Turbocharging

Today

High-performance automobiles have no right to abuse our environment. No individual has the right to litter the atmosphere with emissions, any more than he would litter the side of the road with beer cans. Everyone living in this atmosphere must exercise a certain level of responsibility toward keeping it clean. Through turbocharging, the high-performance, emissions-compatible automobile of today has increased performance more than any other class of vehicle from any era. This situation is not coincidental.

The response of the automotive engineering community to federal and state emissions laws has created a set of controls with such exceptional technology that today's powerful street car can achieve more mpg than yesterday's econobox, and today's econobox can often outrun yesterday's supercar. Good technology applied to an urgent problem, with the performance car enthusiast constantly pushing the envelope, has resulted in a fleet of vehicles that perform better, are more economical, last longer, require less maintenance, don't pollute the environment, and are just downright fun to drive. What did technology do to turn this trick? They invented new equipment. They optimized it well and calibrated it within tight limits. They manufactured it under such control that it is hugely durable. No doubt whatsoever exists as to the extreme durability of electronic engine-management systems relative to breaker-point distributors and carburetors. The technology developed to contend with today's needs consists primarily of electronic fuel injection, programmed ignition-timing control, oxygen-sensor closed-loop feedback, and catalytic converters.

Fig. 161. The best-known 0-60 time for a turbocharged VW is name where under 3.0 seconds.

The combination of these four items is the heart of obtaining the superb driveability and economy we need while keeping emissions within necessary limits. These items are all available in the aftermarket. It is technically feasible to use these pieces of equipment, tune them carefully, and create a fully certifiable vehicle within operational requirements. The first place to start is learning the rules. The most stringent rules are those of the California Air Resources Board (9528 Telstar Avenue, El Monte, CA 91731; (813) 575-6800). It makes sense to play by the strictest set of rules. These rules are available upon request. Secure them, learn them, and let them be the guidelines under which designs are created.

The Future

These are exciting times for car performance. Engineering, quality, performance, economy, emissions, and durability are all experiencing great improvements. It seems as though we get acquainted with a great new model only to have another come along in swift succession, rendering the first one obsolete-Predicting the path of development that turbo- and engine-related systems will take then becomes both timely and precarious.

If it all happens the way I think it should, much work will be done in three distinct categories: the turbocharger, turbocharger system-related hardware, and the engine proper.

Turbocharger Improvements

Virtually all improvements to the turbo will be aimed at forcing it to leap up to boost-producing speeds in less time, If a turbo could be made instantaneously responsive, the shape of the torque curve of a normally aspirated engine and of a turbo engine would be essentially the same. That is the goal. While it is not yet quite possible to achieve that, progress will come in two primary areas: bearing losses and variable A/R ratio turbine housings.

Bearing losses. Power wasted in the bearings of the turbo is simply the drag loss in shearing the oil film in the bearing as the shaft rotates. This loss is proportionately large at low speeds, when little exhaust gas energy is available to drive the turbine, but wanes to minor importance at high speeds. At high speeds, enough exhaust gas energy exists to kick the turbo so fast as to scare most journalists. The actual power lost in the bearing area is, however, enough to mow your lawn. If this power loss could be applied to revving the turbo up quickly from low speeds, the rate of acceleration of the turbine would be considerably greater.

Low-friction bearings can come about in three ways: smaller-diameter shafts, ball bearings or air bearings. Each approach has problems. Smaller-diameter shafts create higher bearing loads and aggravate critical vibration frequencies. Good engineering will be required to make them work.

Ball bearings hold great promise for low friction. The extreme quality control required of a bearing to operate at turbocharger shaft speeds is not fun for a manufacturing engineer to contemplate. It can be and is being done, and one day will be here for us to use, The willingness of some automakers to spend an extra twenty-five dollars per car for an improvement of the magnitude of low-friction ball bearings in the turbo is a situation that is more likely every day. Performance is now as competitive as any other aspect of the automobile.

Fig. 16-2. This cross section shows details of the two oil-wick-lubricated ball bearings of the Aeroecharger.

Air bearings may see use in select applications where cost becomes less a determining factor. The technology of air bearings is well established, but quality control again becomes a huge barrier to volume production. These are the lowest-friction bearings of all and would yield substantial performance gains.

In view of production technology in the world today, I'll vote on ball bearings as the next bearing system for the turbocharger.

Variable a/r ratio turbine housings. All other things remaining the same, the smaller the A/R ratio of the turbine housing, the lower the rpm at which the turbo will produce boost. This same low A/R turbine housing will cause increasingly large exhaust gas back pressure as total exhaust flow rises with increasing rpm. Big A/Rs make large amounts of power because of reduced back pressures but are not exactly splendid for low-speed response.

While not yet commonplace, turbocharger are in production with a design feature that permits the turbine housing to act like a small A/R at low speeds and a large A/R at higher speeds.

Fig. 16-3. Closing one of the two ports creates a small A/R ratio, improving low-speed response. The gradual opening of the second port at higher speeds creates a larger A/R ratio.

This feature is generally referred to as the variable A/R turbine housing. It indeed offers the merits of both large and small A/Rs, all in the same package. With this feature, the turbo comes much closer to the Instant response we want. It also acquires the ability to produce a torque curve similar to a larger, normally aspirated engine at low engine speeds. Two types of variable A/R units are likely to see some form of popularity, The relatively simple twin scroll idea is an inexpensive mechanism that may prove adequate when judged on its own merit. The other mechanism is the VATN (variable area turbine nozzle). The VATN so far outshines all other possibilities that it will prove to be the winning ticket,

Twin scroll turbine housing. The TST housing derives its name from the geometry of the exhaust gas inlet into the turbine. Two different-sized scrolls are generally used, a primary and a secondary. Typically, the primary is open for low-speed operation, and both for high-speed use. This creates the ability of the TST to be a small A/R housing at low speeds and a large A/R at higher speeds.

TST designs are of merit in that they offer a better combination of low-speed response and high-speed power. It would be difficult to configure the unit to control boost by effectively varying A/R. A wastegate is therefore still necessary to control boost pressure. Simplicity of the twin scroll turbine housing is its big selling point.

Variable area turbine nozzle. The VATN is a whole new deal. The vanes of the VATN pivot to present varying areas to the discharge stream, changing; the exhaust gas velocity as it enters the turbine, permitting the speed of the turbine to vary. The merit of the VATN lies in several areas: it acts like a small A/R when asked to do so, a large A/R when required, and it produces a smooth transition through all points between the two extremes. The VATN can create such a huge A/R that turbine speed over the entire range of operation can be controlled by varying the A/R alone. Thus the VATN becomes its own boost control, and no wastegate is required. When no wastegate is present, all exhaust gas energy is available to power the compressor, and 'waste' becomes a thing of the past.

Fig. 16-4. The fastest-response turbo in the world is the VATN Aerocharger.

Fig. 16-5. Details of the VATN, When the nozzles are nearly closed, exhaust gas velocity is high. When they are open, velocity, and therefore back pressure, is lowest.

Fig. 16-6. Response time of the VATN versus standard turbine response. The time required by the VATN is approximately half that of the standard turbo.

Turbine performance can take on whole new dimensions. Since turbine speed is always controlled by the VATNs, the A/R ratio is always the largest possible for the boost pressure at that instant. If the A/R were smaller, turbine speed would rise, creating more boost, which would raise turbine speed, which would raise boost again. This situation will always keep exhaust gas back pressure at its lowest for any given boost pressure. This creates the wonderful condition of the exhaust back pressure being less than the boost pressure. When this 'crossover' occurs, power production takes on new dimensions. This condition is not generally feasible with conventional turbos

without the turbine's being so large that it becomes unresponsive at low speeds.

The success of the VATN is directly attributable to having the vanes in the right position at the right time, which depends on the 'intelligence' of the vane controller Varying load conditions will require the controller to create the correct A/R for exactly that situation. The load condition of steady-state cruise will want the vanes fully open for the least possible back pressure. On application of throttle, the controller must anticipate the pending demand for boost and close the vanes, so as to bring the turbine up to boost-producing speeds as quickly as possible. Once the desired boost level is achieved, the vanes will gradually open as engine speed rises, in order to control turbine speed and thus the boost pressure. Sufficient range of motion for the vanes must exist that the engine redline can be reached before the vanes are fully open. It is clear, then, that the VATN controller is the secret to the extreme benefit of the VATN concept.

Fig. 16-7. This cross section shows the complexity of the marvelously crafted and engineered Aero-charger. Complexity is the trade-off for the Aerocharger's extraordinary response.

Selection of turbocharger size. Few of today's turbocharged automobiles are equipped with the proper-sized turbo. I remain convinced that the reason for this is the incorrect perception by the marketing types as to the desires of the end user. I contend that the end user wants a powerful automobile, not necessarily one that makes boost at the lowest possible rpm. When, and if, major manufacturers size the turbo for the enthusiast, we will see an increase in power, a decrease in charge temperatures, smoother driveability, and an overall increase in margins of safetyall of which is possible with simple changes in size.

Ceramic turbines. A dramatic reduction in the rotational inertia of the turbo can be brought to bear by the use of ceramic instead of metal for the turbine wheel. While a wonderful idea, yielding a tangible improvement in the turbo's response, the ceramic turbine remains expensive and fragile. This writer believes that other than for brief field tests, the ceramic turbine is a feature for the twenty-first century.

Fig. 16-8. Lighter-weight ceramics offer the difference illustrated by curves T25 Ceramic versus T25 Metal

Fig. 16-9. The ceramic turbine will need to prove its long-term durability for its better response to be an overall benefit.

Composite materials. Carbon composites have tremendous strength and stiffness-to-weight ratios. The possibility of compressor-wheel inertia reductions brought about by composite materials seems likely. Further reducing the inertia of the lowest-inertia component of the turbo is perhaps worthwhile. But fixing the weak links first has an element of logic, and the compressor wheel is not the weak link.

General refinements. Without the fanfare accorded revolutionary components, most of the items inside the turbo will continue to be improved in both efficiency and durability. Bearing losses will creep downward, rotational inertias will decrease, heat rejection will improve, and turbine and compressor efficiencies will slowly but surely improve. Steady improvements, but no great changes.

Fig. 16-10. Perhaps the turbocharger of the future will take the form of this innovative design, with variable area turbine control and an axial-flow compressor ahead of the radial compressor.

Turbocharger System-Related Hardware

Intercooling. Although the science of intercooling is well known to automobile designers of the world, the next few years should show large improvements in this area. The force behind the improvements will be a change in attitude. When one of the world's great car companies builds a vehicle they call a Super Coupe and places an intercooler in a position such that the only cooling air it can possibly get must first go through the cooling system radiator and the AC condenser, this is evidence of an attitude problem. It is possible that the refrigeration-cycle gas intercooler may one day be practical. New processes and techniques will be required as AC compressors consume more power than better intercoolers can offset. The part-time intercooler, where the air charge is sent directly to the engine at all boost pressures below the threshold of need for intercooling, may one day yield a tangible improvement in response for the entire system,

Boost controls. Smarter wastegate controls can produce more responsive turbos as well as flatter torque curves. While ultimate power may be little influenced, a torque curve with one or both ends raised a bit will produce a faster car. Electronic control of the wastegate actuator signal will be the concept that permits these improvements. Conventional wastegates crack open at a point well below the desired boost and then creep to the position required to control boost pressure. This early creep robs the turbine of useful energy with which it could gain speed fasten. Having the wastegate open and bypassing substantial energy around the turbine when the turbine is trying to gain speed is basically madness. Electronics will fix that situation. Raising the low end of a torque curve or smoothing a flat spot in it can be accomplished by programming boost signals. A boost-pressure equivalent of an extra passing gear could he programmed as well.

Exhaust system. Virtually all current production exhaust systerns are excessively restrictive. Tailpipe-induced back pressure is truly useless. I do believe it possible to produce quiet, low-back-pressure tailpipes as economically as the current bad designs. This would permit the same turbo systems to operate at the same power with less heat, less boost, and a far greater margin of safety.

The inverted-sound-wave silencer is an idea whose time is yet to come. The principle is to record the sound from the engine, electronically invert it, and play it back, superimposed on the original. The hope is that the two sound waves will cancel each other, eliminating the need for a muffler. Back pressure could be way down, so we remain interested.

Staged and Staggered Turbos. Many interesting schemes have been created to couple two or more turbos together. The purpose is generally to achieve greater efficiency at extreme boost pressures or to gain low-end response and broad, flat torque curves. Such schemes may be useful in vehicles like the fabulous Porsche 959, but the likelihood of such complexity's reaching the market in significant numbers appears small, The value of a hobbyist's trying to recreate such intricate equipment seems staggering indeed. Complexity and cost of this magnitude, accompanied by all the service and repair problems inherent in that complexity, is mind-boggling. Stick to fundamentals, do them very well, and let performance be the staggering factor.

Engine

A clean-sheet-of-paper engine designed specifically for turbocharging would not be dramatically different. However, many details would change:

In my view, the positions of the turbo and catalytic converter would be reversed, to improve cold-start emissions. Turbine response would ordinarily suffer in this position, but the VATN turbo would more than restore any lost response.

Fig. 16-11. The Chevy/Ilmor Indy engine was designed from scratch as a turbocharged engine. The camshafts, port sizes, compression ratio, bore/stroke ratio, and rpm operating ranges were all configured to complement the turbocharger.

Engine speeds would likely be reduced, With the broad-band torque increases offered by the turbo, high rotational speed is no longer necessary to make adequate power. Lower engine speeds would reduce component weights and friction as well.

With lower engine speeds comes the ability to take advantage of longer-stroke, smaller-bore engines. For reasons buried in the foggy depths of thermodynamics, longer-stroke engines can enjoy greater fuel efficiency,

Smaller intake and exhaust ports would improve low-rpm torque by creating higher intake-air velocities, permitting better cylinder filling as a result of the increased momentum of a faster-moving column of air. The turbo will take care of torque for the remainder of the rpm range.

The number of cylinder-head-to-block fasteners should increase. A greater number of smaller studs, perhaps six per cylinder, would be allowed by reduced port areas.

- Heat will be dealt with by at least two changes. First, oil spray onto the bottom of pistons or oil passages through pistons can greatly improve piston strength by reducing operating temperatures. Second, the exhaust port will be insulated to reduce heat transfer to the head and the coolant. This will improve catalyst light-off (response time for the converter to achieve minimum operating temperature from a cold start), reduce radiator sizes, and carry more heat to the turbine for response benefits.

Electronic controls will play an even greater role in running the turbo engine. Today's engine-management systems will be expanded to control VATNs, boost pressures, and, of course, the relation of spark timing and fuel mixture as they influence engine knock.