None of these posts and replies are actually mine. But for the sake of the data, I migrated them over.
Ok, so Jon motivated to make this into a thread. I have been considering doing a mild "street" ALS for a while, and I have always been throwing around different ideas. I've tried to do plenty of searching online but truthfully, there is very little info available.
What is an Anti-lag System (ALS)?
- ALS, also known as "bang bang," is the system that gives rally cars their characteristic pop and crackle when they left off the throttle. It should not be confused with a "launch control" antilag, which is really just a 2 step with timing retard. The purpose of a true ALS is to keep the turbo spinning as fast as possible at all times, the idea being, as the name implies, no lag. This involves keeping the turbo spinning after the driver lets off the throttle. ALS has widely used in rally, because there it is most useful. Big turbos, small engines, and tiny restrictors mean the turbo must actually make more boost at lower crankshaft rpm than higher up in the power band, to flow the most amount of air possible through the restrictor.
How is it implemented?
- ALS has many different implementations, but they are all a means to the same end. Some systems work by bypassing air through the throttle body. This can be done by jacking open the throttle (never really closing it) or using what is basically an oversized ISC. Now that we have air, fuel can be injected. Timing is retarded drastically during this operation to do two things: 1. light the mixture in the exhaust manifold, providing plenty of hot exhaust gas to keep the turbo spinning up and 2. reduce engine torque (good, because we aren't on the throttle anyway).
The other way is to bypass the engine altogether, and route post compressor air straight to the exhaust manifold, in effect making the turbocharger into a gas turbine. This was done on the Audi Quattro rally car of the 80s, Toyota Celica GT4, Mitsubishi Evo 4+ (hardware can be found on the non USDM models) and I believe most modern WRC cars. This system is also pretty simple: a valve between intake and exhaust, a one way check valve to prevent exhaust flow into the intake, and a solenoid connected to a port of the valve so that the system can be under ECU control.
So the air can come from before or after the throttle plate. Fuel can come from anywhere too. You could inject fuel normally (what is normally done), you could potentially have a fuel injector in the exhaust manifold (or a seperate chamber), or you may not inject fuel at all, as was done on the Prodrive P2. Since turbocharged cars are run much richer than stoich anyway, we already have plenty of fuel - air is the key to ALS. You can retard the timing, igniting the mixture in the exhaust manifold, have a seperate spark or glow plug, or no spark at all (again, Prodrive P2, which utilized the temperature of the exhaust manifold to ignite the mixture).
ALS can be from mild (P2) to wild (WRC). WRC systems can generate tremendous amounts of boost, mild systems not so much, but still enough to make a difference. Spool is the enemy. However, mild systems will last a whole lot longer than a WRC system.
Advantages:
- Throttle response
- Broader/more linear powerband
Disadvantages:
- Heat! Lots of it
- No vacuum means no power assist for the brakes (this can be remedied however)
- Reduced engine braking
Other options:
- There are other options to reducing turbo lag. Besides the obvious, like running a smaller hotside, increasing displacement, running a twin scroll, twin turbos, etc., there are a few other options.
- Nitrous: Simple, effective, but you have to have a tank on board and fill it up. Not a continuous system
- Air impingement: A trick learned from the DIYGasTurbine group. This is how they start their turbine engines; leafblowers. An air tank with a few hundred psi with a nozzle spraying the compressor wheel would do the trick. Could be continuous with an air pump on board.
- Electric motor: Some kind of motor that would essentially turn the turbo into an electric supercharger when the engine couldn't provide enough air to keep it spinning fast enough.
Why I'm interested:
- Its exciting! Lol. Besides the obvious fun of shooting flames out of your tailpipe, I've never really seen a road car with an ALS before. So that means I want to try. This system really won't help much at the dragstrip, since we already have a 2-step and NLTS, this is better suited to track or "spirited" road driving. I decided against the other options because I didn't want the added weight and parts of carrying around a tank, and an electric motor would sort of defeat the purpose. I already have gasoline in the car, and I don't mind if I use a bit more of it, why not? A working system would improve driveability, especially on a big turbo car.
What I propose:
- I recently ordered an Arduino and a LCD display. The plan is to build it into an EGT/Turbo tach gauge. I will simply buy a thermocouple for an EGT, and the turbo tach will be built with optics. The idea is to shine a laser or LED on the compressor nut. If the compressor nut is painted black/white/black/white, it will reflect the beam, say, twice per revolution. You can pick up this reflection with a photodiode, and you have a frequency that is twice turbo rpm. Not my idea - I got it off the DIYGasTurbine group on yahoo (they are pretty creative...and destructive). Once the gauges are working, I will start working on an air valve to allow air from the intake to the exhaust manifold. I was originally planning on building a seperate combustion chamber with its own fuel and spark source, but this seems simpler. The air valve can be PWM controlled (maybe use a fuel injector? Probably doesn't flow enough) or a simple valve (like an EGR) that could be controlled by a solenoid. The system would be electronically controlled by the Arduino or ECU and incorporate safety factors, such as switching off the system if turbo speed or EGT were too high. It would also be easy to have a switch inside the car to activate the system. This would also make possible something like a 3D map, with maybe boost, TPS, and target turbo speed. A closed loop system is even possible - but I get ahead of myself...I need to make the gauges first.
