Detonation Control

Rich and Retarded are Crude Ways To Keep Engines Safe

by Kevin Cameron

Detonation, or knock, defines the upper limit of power for spark-ignition engines, for almost everything we do to increase power tends to foster this form of abnormal, often destructive combustion. Once detonation begins, some operating variable must be adjusted to stop it or engine damage will result. Normally this process is what we call tuning. The engine builder, relying on experience, sets the compression ratio as high as detonation will allow. Ignition timing is pushed until the subtle signs of deto are seen on plugs or pistons. Jetting comes down, power comes up, and trouble approaches. Near the limit, this is a tricky business. Maximum power lies right at the edge of detonation. Making engines live reliably there requires race engine tuning experience. All tuners know that when you have to crowd the limit, even the most experienced can make mistakes. Deto erodes pistons and heads. Ring lands collapse. The increased heat transfer that even moderate detonation causes raises engine temperature. In the worst cases, pistons break up and the resulting junk is blown into the exhaust pipe - wrist pin and bearing, twisted rings, together with gritty lumps of what was once the piston.

Only racers need power so urgently that the above risks are worth taking. For production sleds ridden by recreational riders, the tuning variables that can take an engine to the edge are purposely set conservatively. Jetting is rich, timing is pulled back a bit, and compression ratio is moderate. For reliability's sake, it's safer to run a big engine on conservative settings than a smaller one, tuned near the limit. The name of this is stock performance, and it makes good practical sense. Let the racers chase power up and down the air density scale with constant tuning changes. The rest of us are happy to run rich and retarded - and just have fun on the snow.

Then along came emissions regulations. When engines are set rich to keep them safe, more unburned hydrocarbons are released into their exhaust. To meet emissions standards and postpone new standards by voluntary compliance, industry has to find ways to do without running engines rich. New ways must be found of keeping engines safe without this traditional richness.

For a time, we believed that Direct Fuel Injection (DFI) would come along to clean everything up. The DFI computer would be like all the experience of the best engine tuners, written down in software to control our engines perfectly. Let the weather and the altitude change as much as they liked - our new DFI engines would take exactly the fuel they needed and no more.

DFI engines do exist, and they are wonderful, but they aren't the only possible answer to the problem of making emissions control compatible with good performance and engine safety.

Yamaha's contribution is their DCS - Detonation Control System - available currently only on the triple-piped SRX700. A detonation sensor on the cylinder head detects the vibration characteristic of detonation and measures its amplitude. Three levels of system response are possible, depending upon detonation intensity. At the first level (light deto) the DCS dash-mounted indicator light at the left side of the panel flashes slowly and the engine control computer retards ignition timing enough to stop the detonation (in the range of 1-3 degrees). If detonation is heavier, a second level of response is triggered. The indicator light now flashes rapidly and timing is pulled back 3-6 degrees. The third stage - really more of a limp-home mode - retards the timing 6 degrees, limits revs to 6400, and continues to flash the indicator lamp rapidly.

The DCS system is automatic, maintenance-free, and cannot be switched off. With such a system, tuning variables can be safely set closer to optimum instead of rich and retarded. If, on a given tank of gas on a given day, the engine knocks a bit on full throttle, DCS will stop the knock and the only loss will be whatever slight power drop is associated with the momentary ignition retard that the system employs to stop the deto.

This is a lot less power loss than would be associated with the normal richer jetting and retarded timing that are the crude old way to keep sled engines safe. Before Y-DCS, jetting, compression, and timing had to be set conservatively enough that harmful deto would never occur. That means tuning the engine to be safe on the worst possible gas, on the coldest, high-barometer day. This means that on an average day, running average gas, your engine will run rich, suffering power loss from the resulting reduced flame temperature. Y-DCS will also be useful to mountain riders, who in the past have had to go light on the throttle at lower altitudes where air density is higher. With Yamaha's system, detonation control is no longer the rider's responsibility.

Detonation Control System (D.C.S.) Features: 1. Industry-exclusive design brings Indy-car technology into the snowmobile market. 2. D.C.S. allows leaner production carburetor settings to reliably produce increased power and allows improved fuel economy. 3. Sensor monitors combustion vibration for early warning signs of possible detonation. - Detonation may occur from poor fuel quality and/or too-lean carburetor calibration. 4. If detonation is detected, ignition timing is adjusted until detonation stops.5. System operates automatically, effectively and reliably without requiring special operator action. 6. Indicator lamp shows if Detonation Control System is affecting ignition timing.

From an emissions standpoint, any extra fuel added by over jetting shows up in the exhaust as unburned hydrocarbon. Y-DCS eliminates much of this because with detonation control, extra fuel isn't needed to lower flame temperature. With closer jetting, the YDCS engine has sharper throttle response, more acceleration and peak, and lower emissions. These improvements are not huge, but they are real. Since the computer is already on the sled, the Y-DCS system costs the price of the cylinder-head-mounted deto detector, the indicator light, a few wires, and your sled's share of the engine control software department's bill to Yamaha.

Detonation detection has evolved from the equipment used to detect and measure detonation on the lab engines used for octane rating fuels. Racing teams then adapted rugged, compact versions of this for use on the track as detonation counters. One two-stroke GP motorcycle team found that its engines could tolerate 90 detonating cycles per lap. If more than that were detected, tuning variables were adjusted. The next step was to couple deto detection to ignition timing, using the engine control computer to run software that could distinguish between detonation and such this as piston slap. Buick did this with their turbo production engine in the 1980s. Now we have arrived at the point that this system is ready to be part of production sleds.

PROGRAMMABLE IGNITIONS
Correct ignition timing is that which causes peak combustion pressure to occur in the region of 13-15 degrees ATDC. If flame speed rises for any reason, peak pressure will occur earlier than this, and vice versa. In either case, there will be some power loss. Flame speed varies with the purity and density of the cylinder charge - it's high when cylinder filling is good, and lower in "flat-spot" regions where the exhaust pipe pumps less air. This means that for maximum performance, it's useful to vary ignition timing as the engine revs up, to keep the point of peak pressure where it has to be for peak torque. This was impossible back in the days of fixed-timing magnetos, but it's easy for computer-controlled ignitions. This is the job of the ignition map - the software chart stored in your ignition computer, which gives the correct ignition timing for maximum torque for every rpm. The original data for this are gathered on a dyno at the factory by testing the given engine model in steps of (typically) 50 rpm, finding minimum timing for best torque at each speed and then recording it. As your engine runs, its computer measures rpm, looks up the corresponding correct ignition timing, and fires the plug at that point, each time the engine fires.

Because flame speed also rises and falls with throttle opening, a Throttle Position Sensor (TPS) may further refine ignition timing. In this case, the ignition map acquires a third dimension - throttle opening.

This is wonderful for a stock engine because it makes available the maximum torque of which the engine is capable at every rpm and throttle position, without compromise. This increases power and cuts emissions and fuel consumption.

Aha, but what if your engine is not stock? Let's say your stock engine's pipe peaks it at 6800 rpm, but now you install a ported cylinder and individual pipes that peak at 7600. Your ignition computer doesn't know these changes have taken place, and so it goes right ahead supplying timing for a stock cylinder and the pipe that is now hanging from the ceiling in your shop. It may even have a rev limiter that cuts off your modified engine before it can even reach peak power rpm. This is like getting medications meant for the man in the next bed at the hospital - it's all wrong. Not surprisingly, your engine feels dead where it shouldn't and at other rpm it makes good power but the plugs show that's it's knocking. This is a mess. What now?

D.C.S. Details: 1. Detonation Control System monitors combustion vibration, throttle opening position and crankshaft timing (degree of rotation). 2. Ignition timing change depends on detonation sensor signal. 3. If detonation is detected, the system reacts in three stages: A) Ignition timing is retarded by 3 degrees; Indicator lamp flashes slowly. B) If detonation continues, the timing is retarded by 6 degrees; Indicator lamp flashes quickly. C) If possible engine-damaging detonation levels still continue, indicator lamp continues to flash quickly and maximum engine speed is reduced to 6400 RPM for the ‘limp-home’ mode. 4. System automatically resets when detonation stops.

Help is at hand, and even better help may not be far behind. Ski-Doo has a hand-held device for reprogramming ignitions either to work with modified engines or to update an existing ignition to a newer specification. In the first case, you would use the programming device to determine and store a new ignition curve as your engine runs on a dyno. Polaris and A-Cat offer a service by which your dealer returns your ignition module to the factory for reprogramming. Different curves would be supplied depending upon the equipment - pipes & etc. - on your engine.

In the motorcycle world, the Dyno-Jet company now offers a service through operators of its chassis dynos by which their proprietary "Tuning Link" software automatically finds optimum values for fuel injection quantity and ignition timing during one or two dyno runs. This makes it unnecessary to perform a time-consuming step-test and manually search out and write down best values. It is obviously possible to imagine such a system for reprogramming snowmobile engine ignitions. Whatever is possible becomes likely.

The benefits of Y-DCS and programmable ignitions could potentially come together in a combined intelligent system called MBT control (Minimum timing for Best Torque). What such a system does is to monitor cylinder pressure during each combustion event, determine where peak combustion pressure is occurring in crank degrees, and continuously adjust ignition timing to keep peak pressure at the desired 13-15 degrees ATDC. If knock is detected, ignition timing is retarded until knock ceases, as in Y-DCS. An MBT control system would automatically find best timing no matter what the fuel, the conditions, or the modifications to the engine. It would be entirely self-acting.
Such a system is not likely to come on the market soon because of the high cost of combustion pressure monitors. These monitors are derived from microphones originally developed to analyze combustion instability in rocket engines. Right now, sensor cost is high because few are sold. People said the same thing about detonation control systems not so long ago, but here's Yamaha's DCS, a market reality. Why? Because a company making detonation sensors can make more money selling thousands to all of us at a few dollars apiece, than they can selling a few fragile lab specials to engineers at $1500 each.

Y-DCS is a strong play from Yamaha because it allows engines to be tuned for power rather than for safety - everyone is going to want this system on his engine. Before Y-DCS, engines had to be tuned to be safe even when running on the stalest Knock-O gasoline, in high air density conditions - rich and retarded. This is "lowest-common-denominator" tuning. Because Y-DCS automatically protects the engine from the worst case, a DCS-equipped engine can be tuned for average gasoline on an average day - with no risk of detonation damage. This is performance tuning. Big difference.

This is just one more step toward the ultimate goal of the continuously self-optimizing engine. Imagine the possibilities.