Expected mpg improvement from EGR etc cleaning?

Not sure what that means, but it's got me smiling.

One thing: getting a car up a hill seems immensely harder than moving it along a level road.

 

Word nerd pounces: I'm pretty sure the etymology of 'immense' suggests something that can't be measured.

So while the extra work getting the car up the hill is large, it might be too easily quantified to really be called 'immense' . You've got the mass of the car, times the acceleration due to gravity, times the sine of the angle of the hill.

 

haha it was just a simple thought. Because of the angle, the exhaust smoke will be more tend to stuck inside (less flow) of a clogged EGR according to Gas Laws if remember correctly

Posted via the PriusChat mobile app.

 

Gases tend to be pretty responsive to pressure gradients though.

 

I'm comparing an engine with NO EGR with an engine with 21% EGR. Power has nothing to do with this. Power is changed almost linearly by increasing RPM. If the air:fuel ratio stays the same - increasing RPM increases the rate of combustion gas not necessarily the temperature.

It's thermodynamics 101: The temperature of the combustion gas should be proportional to the calories burned divided by the thermal mass (or specific heats) of the chemical components in the combustion gas.

 

As most everyone here increasingly understands given the prices of gasoline these days, you don't get any power that wasn't in the gasoline you burned.

If you need the engine's full power for the current driving conditions, you will get it, and there will be no EGR (it isn't used at full power).

If the driving conditions allow you to get by with 21% less power, the car with no EGR will accomplish it by throttling back the intake and injecting 21% less fuel. The car with EGR will add EGR to the intake, and inject 21% less fuel. Both cars in that circumstance will be burning the same fuel and making the same heat.

Nothing wrong with thermodynamics 101, but to apply it usefully, you also need to pay attention to what's going on.

 

Yeah I hadn't considered throttle losses. So a way to allow 21% less air is to add 3 psi of vacuum to the CC according to the ideal gas law?

 

Same fuel but different volume When the engine is operating at partial loads, by using EGR, the pumping losses are reduced and fuel efficiency improved. So with EGR a little less fuel volume. What puzzles me is that with EGR there will be less oxygen which is vital component. Perhaps the impact of pumping loss reduction is geater than ”loss of oxygen”. Not by large but enough to make difference

 

Agreed; that's what I was hinting toward in the last three paragraphs of #15. A slight reduction in fuel volume, but only as much as you can account for with the marginal efficiency gain from running the engine that way.

 

The engine displacement is effectively reduced?

 

Yeah. What really puzzles me is that amount of oxygen is significantly reduced. On the other hand anybody who has kickstarted something know how much engine fight back i.e. pumping loss. And how it get little easier when opening the throttle like it happen with egr

 

No, it’s like trying to suck air with your mouth when teeth are together versus being in thin air and breathing with mouth wide open. Lungs have to work less with the later. Example i just made up

 

Yes, it's effectively reducing the engine displacement at a cost of added pumping losses.

 

I dont know. You see, with diesels the percentage can be as high as 50%. And egr reduce (not add) pumping loss. That is the whole point.

interesting subject. Funny that in our gasoline Kia there is egr as well and 270000km on the clock. Never even thought about it nor cleaning it

 

Agreed: a little less fuel volume using EGR because of more power extracted from same fuel. But back to my main point: even if the fuel injected was the same, with the EGR case you are using this same amount of fuel to heat 20% more cylinder gas. This lowers not just peak temperatures but average cylinder temperatures as well by a larger percentage.

 

It's still the "less compared to what?" question, though.

You can think of it this way: gasoline has so many kWh per gallon. If the driving conditions say you need y kW of power out of the engine, and engine efficiency is (something near) 0.4, then you need to burn fuel at rate (something near) y/0.4. You have no choice.

There is a stoichiometric ratio telling you how much oxygen you need, to go with that much fuel. The ECM is going to give you that much oxygen, one way or another. Either you have the right amount of oxygen because the EGR has replaced some of the air, or you have the same right amount of oxygen because the throttle has closed further and there is less air.

So there isn't a difference in the amount of oxygen present in those two cases. There's a difference if you compare with-EGR to without-EGR-but-same-throttle-opening, but that just isn't what happens.

Again, when I treat the engine efficiency as simply 0.4 above, that's for brevity. The actual number isn't fixed, and it's a smidge lower in the without-EGR case than the with-EGR, but that matters more for other questions than it does for the one in this post. (Of course it matters directly to the title of this thread.)

 

Got it. Still, the mixture is so different from normal combustion which also make me wonder. Normally there is fresh air + fuel but with egr there is fresh air + fuel + exhaust (CO2, CO, and ”other gasses”) but same stoichiometric ratio is applied? Reaction not influenced by those other (additional) components? I just learned that with gasoline engine egr can only be max 20% so there is not that much exhaust but still.

 

With less oxygen you're going to need less gas. And greater throttle opening if needed, for sufficient oomph.

 

yep, of course. But that oxygen is replaced by something else. That does not influence reaction? Like make it weaker. Or its just about fuel-oxygen ratio regardless of what ”filler” there is in?

 

Think of it this way: the "given" that you start with is how much gasoline you must use. That's where the power comes from, and the ECU has computed how much power you need.

Starting with how much fuel you need, you then compute how much oxygen you need. There's a known ratio for that.

One way or another, you will supply a cylinder charge containing that much oxygen.

You don't get to change how much oxygen air contains. That's a known proportion. So you know how much air you will need, to supply the right amount of oxygen. One way or another, you will charge the cylinder with that much air.

One way: you will charge the cylinder with that much air and nothing else. You will have reduced the throttle opening so only that much air gets in.

Another way: you will charge the cylinder with that much air and some exhaust. Giving you the same amount of air. And therefore the same amount of oxygen.

The exhaust you let in hasn't replaced air. It has replaced vacuum.

That's why the sensor that is used to prove out the EGR flow can be a pressure sensor, detecting the reduction of manifold vacuum when the EGR is applied.

It's also one of the reasons the engine is working less hard in the EGR case: it isn't pumping against as strong a vacuum.

The composition of the exhaust does have some small effects on combustion, what you might consider as the next order of approximation, after getting the basic stuff down. Some of the many other EGR threads around here have links to non-paywalled papers where you can read those details. That includes why the change from pre-catalyst EGR (Gen 3 Prius) to post-catalyst EGR (Gen 4) was not just some engineers looking for a way to clog the cooler less. The industry had been researching that change for years, and because the catalyst changes the exhaust chemistry, the change of EGR source does entail changes in those next-order combustion effects.