From: Rad Davis Date: Fri, 28 Oct 94 14:15:00 UTC To: corvair@shadow.mitre.org Subject: re: EGR Ken Rolt wrote: The following discussion came up on one of the USENET groups. It's interesting and, perhaps worth discussion or modification for our Corvairs, both for emissions reduction purposes (and hence passing an inspection with more room to spare) and also for reasons to make our engines run somewhat better (by better combustion). Comments ? - Ken Rolt I thought I mentioned this when I first joined the echo. Oh well. I guess if the guys on the USENET group are talking about it, it gets more attention. There is definitely some confusion in the USENET posts between manifold heat riser (hot normal air for the engine intake) and EGR, which is exhaust gases being inhaled by the engine somewhere after the throttle valve. Contrary to popular belief, modern intake heating systems don't completely "turn off" when the engine reaches operating temperature. The manifold on port injected water pumpers is usually constantly heated around the throttle valve to prevent icing and the intake air is a blend of hot air from above the exhaust manifold and cool air to achieve a regulated intake temperature of usually about 190 F. Post-1968 carbureted cars also usually have a regulated intake air temperature of about 190 F. In both cases, it reduces the number of variables that the fuel system has to deal with, and increases likelihood that the engine will be producing its certified exhaust emissions. One of the fun paradoxes of cars is that if the air/fuel mixture temperature is lower, more can be packed into a cylinder, so more power is made. But most efficient combustion occurs when the fuel is completely vaporized into the air, and elevated mixture temperatures will raise compression pressures, contributing to more complete and efficient combustion. So ideally, you want to heat the manifold and mixture at part throttle cruise, and cool it at full thorottle for max power. Carbureted cars also usually have some form of heat (coolant on newer engines, frequently exhaust gases running through a passage on older ones) applied to the bottom of the intake manifold to encourage fuel vaporization. This improves transient response, part throttle fuel economy, and emissions because it reduces the tendancy of liquid fuel to puddle inside the manifold. Obviously, port-injected engines don't have this problem. EGR is a whole different thing. It was initially introduced to cope with California's standard for oxides of nitrogen (NOX). Nitrogen's oxides are the principal cause of photochemical smog--that hazy brown stuff Los Angeles is famous for. Unlike CO and HC emissions, which are generally reduced by running an engine in a better state of tune, NOX actually increases as combustion efficiency increases. Nitrogen is the largest single constituent of earth's atmosphere. It normally forms a very stable diatomic molecule (N2) and is usually regarded as pretty close to inert in this form. However, at extremely high temperatures in the presence of free oxygen, the oxygen can break apart the diatomic bond and attach to an atom of nitrogen. Thus NOX is formed. As you might expect, one sure way to reduce NOX emissions is to run the engine rich, so that less free oxygen is available. Unfortunately, this increases HC and CO. The challenge is to reduce the peak temperature of combustion below the point at which a lot of NOX is produced, but without altering the chemistry of the combustion process itself, so that the fuel is still burned quickly and completely. This will automatically reduce the power available (as pressure) from combustion, since T in the ideal gas equation (PV=NRT) is reduced. But the temperature limit of NOX production is still quite high, so it's not much of a loss on an 87 octane street car anyway. One popular way of reducing peak combustion temperature is by water/alcohol injection. It works well, and also keeps the combustion chambers clean, but you have to keep filling up the reservoir. Automakers elected instead to use the engine's own exhaust to dilute the incoming fuel/air mixture-- EGR. Because less fuel is in the combustion chamber, less total heat is produced. But the right amount of air is present for that amount of fuel, since the exhaust gas was introduced downwind of the fuel system, so the fuel is burned completely. Also, compression pressures stay high, or may even increase, due to the elevated temperature of the recirculated exhaust gas, which encourages good, quick combustion. But because less fuel is in the cylinder at combustion, less heat is made, and, if things are adjusted right, the peak temperature of combustion is lower. There's another side benifit. If you do EGR during a light-load highway cruise situation, the efficiency of combustion can be increased, because you are filling the cylinder more completely, which gives higher compression pressures, and thus better combustion. This is worth a couple of MPG. Still another benefit is that knock is caused in part by excessive combustion pressure. Since EGR limits the maximum temperature (and thus the maximum pressure) of combustion, it acts as an effective knock reducer- -so you can run a higher compression ratio, more boost, or whatever. This is one of the tricks automakers use to get the notably higher specific outputs of today's engines versus those of 20-30 years ago. So EGR is not all bad. In fact, you can think of it as being a variation on water injection, but with some other disadvantages and benefits. Water injection won't improve your part-throttle fuel economy, and EGR reduces the cylinder filling efficiency of the engine at full throttle. Both systems have their uses. Rad.