The Technical Questions Thread

[group c n b man]

Is it true that direct injection petrol engines cannot produce exhaust flames on the overun because of the fact that the fuel goes straight into the combustion chamber? No fuel pools around the inlet ports. I read this in the 'ask the experts' section of EVO several months ago. However I wonder if this could be right. For example the Seat Leon 'FSI' touring car seems to produce lots of flames on the overun sometimes. Unless ofcourse it actually has multipoint injection and it just doesn't say it anywhere?

[2ndclasscitizen]

There's no reason why they can't, because the flames are from unburnt fuel escaping past the exhaust valves into the pipes. Fuel pooling around the inlet valve has nothing to do with it.

However, the idea behind direct injection is to try and ensure that all the fuel injected into the cylinder is burnt by more accurate timing and location of the injected fuel. If DI road cars are going about dumping unburnt fuel and shooting flames, it sort of defeats the point (race cars shooting flames are due to their fuel and ignition mapping being pushed hard, and isn't really applicable to road cars).

[LeonOfTheDead]

2cc is right, no reasons why it can't happen, even if it shouldn't happen in first place.
Given this happens when a car is pushed really hard too, I wouldn't be surprised if the electronics would allow it also on DI sportscars, as in those moments environment and mileage aren't the first things to pop into your head, either being the driver or someone watching it from the outside.

[Matra et Alpine]

Engine management on comeptition turbo'ed cars may chose to inject fuel during over-run so that it specifically burns IN the turbo keeping it "spun up".
Dont' know if the FSI does this or not and woudl need to see video to jdge whethe rit might be the case.

[Commodore GS/E]

Today's production engines are very efficient and rather eco-friendly (well, most of them) while delivering a rather high power output. But then again, buying a new engine (only the engine, not the whole car!) is awfully expensive.
Imagine you were the owner of a, let's say, 80s Chevy Caprice. You like the looks of the car, and you refuse to have anything but a V8 in your car. But the horrible mileage and the lack of efficiency begin to bother you. Now here's the question: you could either work on the engine to make it more efficient or simply buy a new production V8. But which would cost less?

[LeonOfTheDead]

What would cost less is quite difficult to answer, but I'd sare to say that the newer engine would surely be more efficient and "eco-friendly" because of many built-in solutions you can't replicate on the older.

[culver]

Today's production engines are very efficient and rather eco-friendly (well, most of them) while delivering a rather high power output. But then again, buying a new engine (only the engine, not the whole car!) is awfully expensive.
Imagine you were the owner of a, let's say, 80s Chevy Caprice. You like the looks of the car, and you refuse to have anything but a V8 in your car. But the horrible mileage and the lack of efficiency begin to bother you. Now here's the question: you could either work on the engine to make it more efficient or simply buy a new production V8. But which would cost less?

In that case the answer is easy, new V8. However, that's a bit of a cheating case because it just happens that the LSx family V8s are easy to buy from GM for a very reasonable price. Of course that only addresses the motor, not the transmission, aero tweaks and other things that would help efficiency.

Ultimately it would depend on the motor. Some times you might be able to update an old motor to make the car more efficient. However, odds are there isn't much you can do economically to drastically improve both emissions and fuel economy. Please note, although it is now popular to call CO2 a pollutant doing so clouds these conversations because CO2 emissions=fuel economy. Other emissions such as SOx, NOx, unburned HC and particulates are rather independent of fuel economy. So while a 1962 VW Bug might do OK on CO2 compared to a Ford F150, it will lose badly when we look at the traditional pollutants.

[LeonOfTheDead]

Please note, although it is now popular to call CO2 a pollutant doing so clouds these conversations because CO2 emissions=fuel economy. Other emissions such as SOx, NOx, unburned HC and particulates are rather independent of fuel economy. So while a 1962 VW Bug might do OK on CO2 compared to a Ford F150, it will lose badly when we look at the traditional pollutants.

qft, absolutely.
Fortunately in EU no one consider CO2 emissions and mileage on the same level.

[culver]

qft, absolutely.
Fortunately in EU no one consider CO2 emissions and mileage on the same level.

Do you mean they no longer call CO2 an "emission" the same way they would call SOx and emission or do you mean they no longer equate CO2 and fuel consumption?

[LeonOfTheDead]

They basically never equated CO2 to fuel consumption, we consider the two things separately, which I think it's a better idea. Even if both of them are influenced not only by the combustion process, they are not so strictly related, especially in real world driving.

Over here pollution is based on CO2, ignoring all the other gases, and taxes or incentives are based on it, no one bothers to check mileage if not from a customer point of view.

[Ferrer]

They basically never equated CO2 to fuel consumption, we consider the two things separately, which I think it's a better idea. Even if both of them are influenced not only by the combustion process, they are not so strictly related, especially in real world driving.

Over here pollution is based on CO2, ignoring all the other gases, and taxes or incentives are based on it, no one bothers to check mileage if not from a customer point of view.

But in official figures they are closely related. More fuel consumption means more CO2 per kilometre, and viceversa.

[henk4]

But in official figures they are closely related. More fuel consumption means more CO2 per kilometre, and viceversa.

sounds logical, as CO2 is a product from the burning of fossile fuels, so the more fuel you use, the more CO2 you will produce...

[culver]

They basically never equated CO2 to fuel consumption, we consider the two things separately, which I think it's a better idea. Even if both of them are influenced not only by the combustion process, they are not so strictly related, especially in real world driving.

Over here pollution is based on CO2, ignoring all the other gases, and taxes or incentives are based on it, no one bothers to check mileage if not from a customer point of view.

That makes sense. Even though they are closely related, a liter of diesel vs a liter of gasoline vs a liter of ethanol will each likely have a different total amount of carbon thus CO2 output after burning. The only time 50% more CO2=50% more fuel consumed is if we are talking the exact same fuel.

I am of course assuming that your HC and CO emissions are low enough to not affect the CO2 numbers. You have to have a really really bad motor to have your HC and CO emissions show up as a reduction in CO2 output!

[CapnBoost]

Consider the cost of the new engine, then subtract your theoretical fuel savings. Also consider that your at highway speeds aerodynamic drag is going to be significant on your '80s vintage flying brick, and will reduce the improvement rendered by your new engine.

That being said, if you're a serious DIYer you could probably rig up fuel injection, lean cruise, and any number of other subtle tweaks that for your investment of time and money could return significant gains.

Ultimately (cost - $ of fuel) will be the deciding factor. Say an Lt1 all the parts and labor to swap it for... $1500 you're talking about >300 gallons of gas maybe 25-30 mpg (if you tweak the loose nut behind the wheel)... maybe a years worth of driving. But keep in mind you're leveraging fuel saved, so if the improvement is only 5-10 mpg (over what you've got now) then it'll take a lot longer for your swap to pay for itself.

[Commodore GS/E]

Another one... as you know, Porsche's new engine is a 3.8l flat 6. The GT3 version has a rather high power output if you consider the displacement and the fact that it's still naturally aspirated. However: when Alois Ruf (who has LOADS of engineering knowledge, especially about Porsche engines) presented the new RGT (that has a V8), he said that the current Porsche engine is the absolute peak of what's doable with a flat 6. In his opinion, the only way to build more powerful NA-engines would be to replace the B6 with a V8 or a B8 (although he complained about the vibration issues on B8's). My question: is he right? SHhould Porsche really switch to a V/B8 and take the risk to lose the 911's character?