The Technical Questions Thread

[Kitdy]

A 2 stroke engine doesn't look anything like a 4 stroke engine.

You may not like Howstuffworks, but it's the best way to show it to you.

Howstuffworks "How Two-stroke Engines Work"

A 2 stroke engine with overhead valves.......where would you put them?

The cylinder itself acts as the intake and exhaust valves. So you wouldn't be able to put them anywhere.

And to answer you question about putting a 2 stroker in a car....

Howstuffworks "How Two-stroke Engines Work"



Also, I didn't know 2 stroke diesels existed. They work the same way I guess, without the spark plug?

I have read these articles - they are somewhat informing but lack specific details and the interaction that UCP offers, although they are more authoritative.

Just because two-strokes don't work at all like four-strokes, doesn't mean that they could be modified at the cost of higher expense and greater weight to have OHC - or even direct injection - for increased efficiency. Am I right about this?

Now, about this whole energy density business. Diesel has a higher energy density per volume, but lower energy density per mass.

In an engine, which of these is the limiting factors - mass or volume?

If I had to guess, based on some rocky logic, it would be mass because the volume of the gas is insignificant as it is mixed with the air - it is the mass of the gasoline in the cylinder that determines how "shit goes down", for lack of a better phrase.

EDIT: After playing around with some numbers for a while, I figured out that gasoline produces more energy than diesel in a cylinder in a typical explosion. It took a while as I had to find out the volume of air/fuel that went into a cylinder, but I believed I figured it out (the volume of fuel in a cylinder is near negligible). This was only a theoretical situation as well, I must add.

[hightower99]

I have read these articles - they are somewhat informing but lack specific details and the interaction that UCP offers, although they are more authoritative.

Be careful to take any information on howstuffworks with a grain of salt

Just because two-strokes don't work at all like four-strokes, doesn't mean that they could be modified at the cost of higher expense and greater weight to have OHC - or even direct injection - for increased efficiency. Am I right about this?

You have to consider the pros and cons of 4 and 2 stroke engines. The Pros for 2 stroke engines are: simplicity, cheap to manufacture, normally more tough and reliable. You do not want to get rid of those pros. BTW seversl modern 2 stroke engines (for scooters) have direct petrol injection to help with emissions and power. You still won't seem them in cars

Now, about this whole energy density business. Diesel has a higher energy density per volume, but lower energy density per mass.

Yes mostly because it is more dense then petrol and is made up of longer carbon chains.

In an engine, which of these is the limiting factors - mass or volume?

depends on what you mean. Volume is obviously the limiting factor as to how much you can take with you in the car. However air/fuel ratios are based on mass so mass is important.

If I had to guess, based on some rocky logic, it would be mass because the volume of the gas is insignificant as it is mixed with the air - it is the mass of the gasoline in the cylinder that determines how "shit goes down", for lack of a better phrase.

Yes and no... Mass is important for determining how much fuel to inject into the engine (engines meter fuel by mass) However the volume of the fuel is never insignificant especially as it is injected as a liquid (high density low volume) but will evaporate to a gas (low density high volume). The thermodynamics of just that process is not insignificant at all.

EDIT: After playing around with some numbers for a while, I figured out that gasoline produces more energy than diesel in a cylinder in a typical explosion. It took a while as I had to find out the volume of air/fuel that went into a cylinder, but I believed I figured it out (the volume of fuel in a cylinder is near negligible). This was only a theoretical situation as well, I must add.

Then you got something wrong, either that or you didn't take some important variables into account. I can run through the math comparing two 500cm^3 cylinders (one running in a petrol engine and one running in a diesel).

[drakkie]

so what do you call the little bits of diesel inside your mist?..see attatchement for one of my sources regarding this precise topic->

haha bugger. Your grasp on English is still a tad bit better I hope you see what I mean, the individual droplets are so tiny and almost completely vapourised that I literally translated the Dutch word. Dictionary gave me "haze".

do you mean combustion begins before TDC due to time taken for fuel to begin releasing energy i order to power piston?

Exactly. See the attached slides.

1) The graph shows the amount of torque against the number of degree before TDC.
2) When running stationary the control over the ignition (knocking) is very important. There is not much power demand anyway. "goede regelbaarheid" = the zone where the best control is. "slechte regelbaarheid" is the zone you want to avoid when being stationary.
3) After a cold start. The exhaust gas temperature has to rise offcourse. To do so the EMS can vary the angle. Graph shows temperature of the gasses vs. angle
4) This shows the way the EMS is programmed to avoid knocking because of too high of an engine. It will always keep a margin to avoid knocking. The size of this margin differs though, the teacher who worked for Peugeot for 20 years, noted that their current margin is about 0.5 degree.

[culver]

HT,

A few years back Chrysler was working on 2 stroke production car motors. My understanding was they just about had the emissions issues licked but ran in regulatory issues. The emissions systems of a 2 stroke motor would be different than that of a 4 stroke. What I recall was the US OBD II rules required certain emissions parts/systems that were not needed on a 2-stroke motor. To fit the parts added costs and made the system hard to justify economically. From what I recall (and this is all recall thus subject to being wrong) the system could basically meet the intent of the law but not the letter of the law.

[drakkie]

Thats a good general rule of thumb. Some smaty pants might bring up odd examples to disprove the assumption, but its a fairly trustable modern trend.

Mitsubishi GDI, FSI systems, the lot.

These systems are only efficient in a very limited range, which is very rarely achieved in normal traffic. To compensate for that they are usually programmed to be able to inject heterogenously and homogenously. So concluding 95% percent of the time they are homogenously-direct injected as well

[jediali]

Mitsubishi GDI, FSI systems, the lot.

These systems are only efficient in a very limited range, which is very rarely achieved in normal traffic. To compensate for that they are usually programmed to be able to inject heterogenously and homogenously. So concluding 95% percent of the time they are homogenously-direct injected as well

yes but in FSI/GDI systems it goes between the two and therefore has the ability to replicate the two. The direct injector in the FSI pic replicates indirect injection when needed, but has capability to perform fuel addition once intake valves are closed.

So yes your right, these engines break the rules a little and are certainly a few steps behind HCCI but they operate on the principle that direct injection is used for heteregeneous operation and indirect injection is used for homogeneous operation.

thanks for pics: I cant beleive your lecturer uses pics from audi..surely he can find better academic sources? can you share that ppt or is it private?

edit: in hindsight, i cant read dutch so...

[drakkie]

yes but in FSI/GDI systems it goes between the two and therefore has the ability to replicate the two. The direct injector in the FSI pic replicates indirect injection when needed, but has capability to perform fuel addition once intake valves are closed.

So yes your right, these engines break the rules a little and are certainly a few steps behind HCCI but they operate on the principle that direct injection is used for heteregeneous operation and indirect injection is used for homogeneous operation.

Exactly.

thanks for pics: I cant beleive your lecturer uses pics from audi..surely he can find better academic sources? can you share that ppt or is it private?

edit: in hindsight, i cant read dutch so...

Your welcome. I have no idea why he used it. It is not uncommon to find picture's from all kinds of companies in our powerpoints though. In this case, i think it was handy because of copyright, he was explaining the basics anyway. On a seminar recently I talked to some students from our competitors and they mentioned they heard from a teacher that sued our school for infringement of copyright. Maybe that's a reason why they look for these kind of sources ?

[NSXType-R]

As this link shows, a 2 stroke diesel has overhead valves. It can even have a 4 valve head where all valves are exhaust valves.
Howstuffworks "How Diesel Two-Stroke Engines Work"

Thanks for the article.

I have read these articles - they are somewhat informing but lack specific details and the interaction that UCP offers, although they are more authoritative.

Just because two-strokes don't work at all like four-strokes, doesn't mean that they could be modified at the cost of higher expense and greater weight to have OHC - or even direct injection - for increased efficiency. Am I right about this?

No idea, you're going to need to talk to an engineer if you want answers there. Sorry.

[Kitdy]

Then you got something wrong, either that or you didn't take some important variables into account. I can run through the math comparing two 500cm^3 cylinders (one running in a petrol engine and one running in a diesel).

So you're saying that diesel fuel produces more energy in an explosion even though it has a less energy density per mass - and the mass of diesel in a diesel engine and gasoline in a gasoline engine are similar?

[Lagonda]

I thought that diesel had a higher energy density ?

[Kitdy]

I thought that diesel had a higher energy density ?

Diesel has a higher energy density by volume, but lower energy density by mass.

[hightower99]

HT,

A few years back Chrysler was working on 2 stroke production car motors. My understanding was they just about had the emissions issues licked but ran in regulatory issues. The emissions systems of a 2 stroke motor would be different than that of a 4 stroke. What I recall was the US OBD II rules required certain emissions parts/systems that were not needed on a 2-stroke motor. To fit the parts added costs and made the system hard to justify economically. From what I recall (and this is all recall thus subject to being wrong) the system could basically meet the intent of the law but not the letter of the law.

The latest I have heard of was back in '92 when Ford and Orbital worked together on a 3 cylinder 2 stroke. They demonstrated it in a Fiesta but apparently there where major doubts concerning emissions performance and durability.

So you're saying that diesel fuel produces more energy in an explosion even though it has a less energy density per mass - and the mass of diesel in a diesel engine and gasoline in a gasoline engine are similar?

Well think about it. Petrol only has 0.7% more energy per mass then diesel yet if both are burning at their stoichiometric ratio then petrol will be mixed at a ratio of 14.7:1 and diesel is mixed at 14.5:1 which means that there will be 1.4% greater mass injected in the diesel cylinder. However diesel engines don't run at stoichiometric, they always run lean even at WOT. However the majority of diesel engines are turbocharged so they still burn a greater mass and therefore release more energy.

These systems are only efficient in a very limited range, which is very rarely achieved in normal traffic. To compensate for that they are usually programmed to be able to inject heterogenously and homogenously. So concluding 95% percent of the time they are homogenously-direct injected as well

Wrong.... Current direct injection systems can run the engine in stratified charge mode in all light load and most medium load conditions, very efficiently. They are programed to run in homogeneous mode at high loads because that is how you get the most fuel and air into the cylinders to produce the needed power. The majority of the time these engines are running in stratified charge mode (heterogeneous). Only during high load and sudden medium-high loads does the engine work in normal (homogeneous) mode.

[2ndclasscitizen]

HT,

A few years back Chrysler was working on 2 stroke production car motors. My understanding was they just about had the emissions issues licked but ran in regulatory issues. The emissions systems of a 2 stroke motor would be different than that of a 4 stroke. What I recall was the US OBD II rules required certain emissions parts/systems that were not needed on a 2-stroke motor. To fit the parts added costs and made the system hard to justify economically. From what I recall (and this is all recall thus subject to being wrong) the system could basically meet the intent of the law but not the letter of the law.

Emissions are a big problem for 2 strokes, simply by the way they work. They burn oil, which doesn't help emissions, and their performance is heavily affected by the exhaust, so putting cats etc on the exhaust will strangle their power output.

[hightower99]

Emissions are a big problem for 2 strokes, simply by the way they work. They burn oil, which doesn't help emissions, and their performance is heavily affected by the exhaust, so putting cats etc on the exhaust will strangle their power output.

It isn't that difficult to make a 2 stroke engine that doesn't burn oil (at least no more than a 4 stroke). And the difficulty with the exhaust is also simple to overcome.


turbocharged 2-strokes FTW.

You could get rid of the crankcase intake/transfer and just have a simple port at the bottom of the cylinder. Add a pressurized oil system just like a 4 stroke and with a simple rotary valve in the exhaust and a turbocharger you don't need to fiddle around with tuning the exhaust. Sprinkle direct injection on top of that, bake in oven at 300 degrees (C) for 5 years and you will have an excellent engine

[Matra et Alpine]

No, ht, just by the way it works you can't get rid of the oil consumption.
Can't use scraper rings so oil will ALWAYS be on the bores.