So, not too long after I had originally bought my TDi last year, I was filling up, and this guy with a big Cummins truck was complimenting me on the new ride, and we got to talking performance upgrades (because I already knew I was planning on a few!
), and that's when he told me about Snow Performance, and showed me the setup he had for his truck. I was initially sceptical, but nonetheless impressed.
Of course my initial thought was "Man, injecting water into a running engine can't be good..." However, having done thorough research into the matter, I eventually warmed up to the idea.
Apparently, the notion of injecting water into an internal combustion engine (both gasoline or diesel) is nothing new. Fighter pilots during WWII injected water into their engines as a means of synthetically raising the effective octane rating of the fuel, and thereby allowing for higher compression ratios, better thermodynamic efficiency, and therefore significantly higher power. During the course of its development, the popular Rolls Royce Merlin V12 engine used in the American P51 Mustang and the wonderfully famous British Spitfire, went from pitiful (for a 27l engine!) sub 1000HP numbers to over 2,500HP (Holy $!!!) towards the end of it's development with water injection. The strangest/coolest thing about it was that despite the higher compression ratios, forced-induction boost pressures, and the enormously increased power outputs, the engines themselves became more reliable as well because they had lower overall operating temperatures.
That's good. Lower temps means less thermal stress on critical engine components, and therefore higher reliability.
So what goes on during the diesel cycle when W/M is a part of the equation? It takes a little time to fully explain, so bear with me as I try my best to go into detail on what happens during each stage of the process (apologies if TL;DR
):
Intake:
For starters, methanol is miscible in water (they become chemically homogeneous via an exothermic reaction (produces its own heat when they mix)). This dramatically affects the evaporative qualities of the water, so that it not only freezes at lower temperatures, but also evaporates into a gas at lower temperatures as well. This is what we want during the intake, because it means that when this is sprayed into the charged air stream (post intercooler, but pre intake manifold), the charge air temperatures will drop even further as the fine mist of W/M absorbs even more heat and becomes more gaseous. Cooler charged intake means a denser charge, and thus more stored potential energy.
Compression:
It's important to remember that the auto-ignition point for most diesel fuel is somewhere in the neighbourhood of 210 degrees C (I think. Correct me if I'm wrong). Already that's more than twice the temperature (at 1 bar) for water to become steam. As the temperature and pressure of the compression rises, the W/M will remain suspended in the charged air without auto-igniting because it has a much higher auto-ignition threshold than diesel fuel. The compression temps and pressures won't be much different than just straight air, but with the W/M, much more of that energy will be stored in the water vapour than in air molecules, because water has a VERY high heat density compared to other compounds. This is good, because it means that there's more heat being absorbed by the water (which can be exhausted) than there is heat being absorbed by the engine itself (like the cylinder side walls, pistons, heads, valves, etc), which can't be exhausted.
Ignition:
In a lot of modern diesel engines, especially in our TDi's, it's not uncommon for there to be multiple, and very precise, fuel injection occurrences. Usually there's a small pre-injection just before the piston reaches TDC (top of the compression stroke). This is done in order to rapidly raise temperatures and pressures right before the main injection at TDC. This is where things really get interesting in the case of W/M. Due to the fact that the methanol ignites faster than the diesel fuel does (which tries to push the piston faster during the power stroke, and thus gives you more power), the pre-ignition fuel injection will cause energy levels to rise even further, while the water will keep temps in check by absorbing that additional energy without yet completely vaporizing. Even though temperatures are now well above water's normal boiling point, the enormous pressure will keep some microscopic water droplets from vaporizing yet. *I'm not entirely sure if the water at this point in the process becomes a supercritical fluid or not, but wouldn't surprise me if it did briefly.
So, now when the main power injection happens, the fuel is going to be injected into much higher energy dense conditions, which allows it to combust more efficiently and completely. As we pass TDC, and the piston moves back down for the power stroke, any microscopic water droplets that couldn't vaporize before, due to the enormous pressures, will now become a very hot and very dry steam as temperatures rise (from the fuel combustion) and pressures fall (from the piston moving down). This steam expands very quickly and with a LOT of force, which in turn provides additional torque.
Exhaust:
Due to the fact that more heat from the combustion process is absorbed by the water than by the engine itself, that heat is therefore more efficiently taken out of the engine when all that steam is vented out during the exhaust stroke. This does mean that you may see slightly higher EGT's but nothing harmful or dangerous. The EGT's are higher because your engine is more efficiently burning its fuel, as well as exhausting the heat from that process, not necessarily because the combustion itself is hotter. This isn't necessarily a bad thing either, especially when you consider that the EGR system can now work better as well. The EGR has more heat with which to burn away excess particulate matter. Plus there's less particulate matter going into it in the first place because the combustion process is more complete and therefore cleaner.
Bonus effects:
One of the biggest issues that has plagued direct injection engines, both diesel and gasoline, is carbon build-up on the intake valves. The reason for this is due to the fact that there's normally a small bit of overlap between the exhaust stroke and the next intake stroke with relation to which valves are open or not. In other words, there are times when BOTH the intake and the exhaust valves are (partly) open at the same time while the remaining exhaust gasses are still going out and the new intake gases are coming in. OK, so what? Why does carbon build up on the intake valves and not the exhaust valves? Temperature. The exhaust valves are going to be much hotter than the intake valves, so any carbon that would build up there, usually gets burned off. That's not the case for the intake valves however. Carbon will build up, but it never gets hot enough for it to be burned off.
Before the advent of direct injection, the intake valves usually had some kind of direct contact with unburned fuel as it went into the combustion chamber. That unburned fuel has a cleaning effect on carbon deposits, which normally keeps carbon build-up in check. With direct injection, that is no longer the case, as fuel is injected directly into the combustion chamber, and so you lose that cleaning effect. With W/M, that cleaning effect is reintroduced, and the carbon build-up issues are mitigated. Think of it like getting your engine steam-cleaned every time you drive it!
So, to sum up, what does W/M do for you?
Keeps your engine clean and healthy (Carbon build up? What carbon build up?)
Improves the thermodynamic efficiency of the combustion process, which equates to:
Reduced emissions
Reduced thermal stress
More power
More torque
Better MPG (I believe Snow Performance's MPG gains are a little inflated, but personally I've noticed an improvement of at least 1 or 2, maybe more, MPG on average)
So, is it worth it?
Well, I got mine installed for about $1,500 including the cost of the kit and the labour for installing it. When you consider that it can easily cost that much (or more!) to have someone take apart the top of your engine to wire-brush, scrub, and clean out all that nasty carbon build-up, yeah, I'd say it was money well spent. I've put about 7k miles on my car since I had it installed, and I have had zero issues with it whatsoever. I'm happy with it!
Hope this has been thoroughly informative and helpful!