Controlled breathing: better air management improves engine efficiency, but at the cost of elaborate controls. (Feature Focus: Automated Controls).
Far in the valley below, an automobile engine labors along, gasping for air like our climbers on high. The slight pressure exerted by the atmospheric column of air at sea level is scarcely enough to supply what an internal combustion engine is capable of consuming.
If, during the climb, you strap to your face a pressurized oxygen mask, your blood would brighten immediately and your tired, depleted muscles would soon revive. The machine that is you needs oxygen to perform.
Turbochargers boost engine performance in the same way that bottled oxygen helps mountaineers climb high.
At the risk of belaboring a metaphor, imagine that with your every breath you inhale and exhale the same volume. That is to say, there's no such thing as deep or shallow breathing.
Under these conditions, somewhere between standing up after a break and ascending the steepest sections of the climb, your lungs reach a point where they are working at optimum capacity.
Though greatly simplified, that is the nature of the internal combustion cycle operating with fixed cams, which open and close the intake and exhaust valves by the same amount and at the same point in the cycle every time, regardless of engine speed, load, or external conditions.
Maybe not for long, though. Engine spark and fuel metering have already escaped their bonds of purely mechanical control; engine respiration will be the last of the combustion triumvirate to fall.
VALVES OPENING ON DEMAND
A fuel-injected engine feeds on a mixture of gasoline and air. By monitoring the amount of air coming through the intake manifold, the fuel control dispenses an allotment of gasoline for efficient burning in the cylinders. In stepping on the gas pedal, a driver in actuality increases oxygen flowing to the engine by opening a throttle plate that sits in the path of incoming air. When he lets off the gas, this plate closes, throttling the engine.
Although proven as an effective method of controlling engine speed, throttling wastes energy. A constricted intake forces the pistons to pull against a partial vacuum, creating pumping losses.
Valvetronic engine technology which BMW introduced in 2000, eliminated a throttle plate and began using the valves themselves to control engine speed. An eccentric shaft that acts upon intermediate rocker arms adjusts the stroke lengths of the valves. A motor moves the eccentric shaft in response to driving conditions.
Engine controls specialists like Anna Stefanopoulou, a professor of mechanical engineering at the University of Michigan; Gregory Ohl, supervisor of advanced power-train controls with DaimlerChrysler; and Sharon Liu, of General Motors' Powertrain Advanced Systems Control, agree that providing engine designers with even greater flexibility to move valves any way they want will improve engine performance.
"Engine designers love degrees of freedom," Stefanopoulou said. The average car engine today has only two, she explained: electronic fuel injection and electronic spark timing.
Along with additional freedoms comes an almost intractable problem, according to Ohl. Yesterday's engine control systems took an empirical approach to telling engine actuators where they should be for any particular set of conditions.
"For given sensor values, the control system would feed forward to where you wanted the engine actuators to be," Ohl explained. The engine was considered a black box, he said. In this approach, every conceivable sensor value corresponded to an actuator position read off a look-up table.
But as sensors and actuators grow in number, the possible permutations they can create expand exponentially, making this control approach unruly, at best, and difficult to manage.
For this reason, next--generation controls are based on models, Ohl said. With these models, control engineers can characterize the flow through an engine, for example. They depend on data from a more fundamental level than that used in earlier empirical approaches. There's more extrapolation. They learn to trust their models, he added.
Think about the tradeoffs a cam has to make on engine performance, Ohl said. A racing cam, in one example, is shaped to optimize engine output at high speeds without regard for the way it roughens up an idle. With camless valvetrains, "We don't have to live with that compromise," he said.
Under a program funded by the Department of Energy the National Science Foundation, and Ford, Stefanopoulou and her research team at the University of Michigan developed and demonstrated a robust control system for a camiess gasoline engine with an electromechanical valvetrain actuator.
Camless valvetrains add six degrees of freedom to engine control, amounting to three per intake valve and three per exhaust valve, and corresponding to a valve's opening, closing, and lift, she explained.
The extra degrees of freedom in a camless engine are both a blessing and a curse for the engine control system, she added.
The blessing lies in the system's promise to eliminate the need for inefficient throttling and in its ability to deliver higher torque. Also, a camless engine could deactivate unneeded cylinders for better efficiency. It could dispense with having to recirculate exhaust gases through EGR systems.
As for its curse, a camless engine could be noisy and susceptible to wear. An electromechanical valve's most likely embodiment would hold the valve between two springs and two counteracting electromagnetics. In moving from the controlled extremes of valve open to valve closed, the valve would enter a region of free flight where neither electromagnet exerted any control over it.
As the valve approached the reach of the electromagnet's field, the control system would quickly have to acquire information about its trajectory to bring in the valve for a soft landing. Otherwise, the valve would slam down into its seat, making noise in the immediate moment and excessive wear over time.
Stefanopoulou adds a few numbers from her research to clarify the point: At 3,000 rpm, each of a test engine's electromechanical valves moves a distance of 8 mm, 100 times a second. With the lab's control algorithm--a fast position feedback with nonlinear gain and an iterative learning scheme based on multivariable control theory (don't ask)--her researchers have been able to propel a valve up to 11 km/hr and land them securely within 1 millisecond. Will that be enough to render camshafts obsolete? The jury's still out on that, Stefanopoulou reported. But, Ford Research Laboratories continues evaluating the algorithm.
In the move away from camshafts, engine builders would replace a single reliable component with a complex system comprising many, many components of a more dubious integrity. A camshaft can break, yes, and bring about catastrophic engine failure. Ordinarily, though, that collection of eccentric lobes ticks away mile after mile without any fuss.
Reliability of the camless engine will have to be built in through a combination of "estimators" and "diagnostic routines," Stefanopoulou said. Sensors observing the engine would detect misbehavior. Then estimators would sift through possible causes. Next, computer algorithms would adjust commands to the engine to rebalance the cylinders and continue delivering torque for the driver. In critical situations, such as a valve gone bad, the controls would need to land the valve safely, deactivate it, and at least enable the car to limp off to find help.
All this would have to happen in less than a millisecond, she added.
A camless engine could represent a theoretical endpoint to an idea that has found its expression at practical, incremental waypoints, such as BMW's Valvetronic system. Another example can be found in Porsche's VarioCam Plus system, which brought variable valve timing and low/high reach intake valves to the 2000 Carrera 911 Turbo.
"Many cars nowadays have variable camshaft timing," Stefanopoulou said. She predicted such systems would become as common in a few years as electronic fuel injection and electronic spark timing are today.
Continuously variable transmissions, or CVTs, started as theoretical visions also, GM's Liu added. Now they are commercial entities. Camless engines may follow the same path, she said.
Nothing is as frustrating as typing a command into your computer and waiting for the screen to respond--except maybe stepping on the gas to pass another car and getting the same you-want-it-when? response from your turbocharged engine. You have to allow time for the turbocharger to process your demand. That drawback notwithstanding, there's probably no better method of coaxing performance out of an engine than by compressing intake air.
Ford Powertrain Research and Advanced Engineering Technical Specialist Julie Buckland is investigating ways of eliminating the drawback of turbo lag in gasoline engines through the use of VGTs, or variable geometry turbochargers. She's also seeking ways to reduce turbo overshoot, another control problem in which the boost overruns demand and a driver gets more torque than he's asking for.
Turbochargers started out as purely mechanical devices that used waste gates to dump excessive boost pressure as a way of preventing engine or turbine damage. Most fixed vane turbochargers today use an active waste gate to add flexibility as to when the gate opens, Buckland explained.
A turbine still requires a certain amount of exhaust gas running by it to produce any noticeable effect when a driver steps on the gas. The time that the driver must wait for the boost is known as turbo lag.
By varying the geometry of the turbine nozzles or buckets, VGT manufacturers provide a means of keeping rotor speed up, and boost pressure available for a range of engine rpms.
Of course, the VGT adds a third actuator to Buckland's controls list that already includes the exhaust gas recirculation system and the throttle. Her job is figuring out how to best coordinate those actuators while receiving information from a handful of sensors that monitor intake manifold pressure, boost pressure upstream of the throttle, delta pressure across the EGR valve, and, sometimes, airflow.
In addition to improving turbocharger response and limiting overshoot, Buckland strives for robustness in the controls so that the system continues to behave properly as miles accumulate. One way of accomplishing this is to use feedback control, she said.
For those of us who can't remember if we even took Control Theory 101, let alone passed it, Buckland offered a couple of what she called "oversimplified" descriptions. Feedback control compares a measured quantity with the desired value and uses the difference to adjust the actuator. Feed-forward control adjusts the actuator based only on the desired value.
She and her associates recently compared throttle and VGT control of boosted direct-injection gasoline engines, examining a number of different controller architectures. One that held a great deal of promise made use of an extra sensor not normally included in production cars that monitored exhaust manifold pressure.
The system uses a feed-forward plus proportional plus integral feedback controller for the throttle based on the intake manifold pressure. For controlling the VGT it applies a similar architecture, but bases the feedback control on a weighted sum of pressures in the intake and the exhaust manifolds.
The combination of feedback and feed-forward control addresses the prime requirements of VGT control. Feedback control assures that the boost demand is met and that any change in the engine inherent to the measurement is accounted for. The feed-forward control assures a fast response.
Over the years, turbochargers have seen greater application in diesels than they have in gasoline engines, primarily because of the necessity to improve the relatively low power density of these engines. VGTs are used there to reduce turbo lag as well.
In gasoline engines, turbo overshoot is a difficult control problem, partly because modern catalytic converters used in controlling spark-ignited emissions make the most efficient simultaneous conversion of hydrocarbons, carbon monoxide, and oxides of nitrogen when the air-fuel ratio stays very near the stoichiometric ideal. A poorly controlled VGT system can throw off this ratio and increase emissions. Alternatively if the ratio is strictly maintained, torque is affected. A diesel, on the other hand, does not use this type of catalytic converter.
In diesels, excess air brought on by turbocharger overshoot and the changing torque that ensues can be controlled by varying the fuel supply Buckland said. In gas engines, the fuel tracks the air, so the option doesn't apply
By the way, we almost forgot. You can take another breath now. Better yet, just breathe any time you want.
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|Date:||May 1, 2003|
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