variable valve timing...

[slick]

I wouldn't want to enter into a technical discussion over the other posts, and so wont disagree with the comments made. However, thinking a little more laterally, could the unburnt fuel ignited in the exhaust be down to the engine managment? Under heavy braking and acceleration (Downshift, Upshift) I was under the impression that brake anti-lock and traction control were activated to stabilise the car. Doesn't this mainly involve cutting the spark to the engine? Hence could also be a source for the flames to originate?

[pyry]

you could actually make the topic the topic and not a sidenote of something relating to the nature of the question. i personally dont want to read a forum with 400 threads by the topic of "dumb question" "dumber question" or"smart question" if you have the urge to apologise you can add a ps. sorry

[johny]

yes i also don't like it, you post a question because you don't know the answer, that's being inteligent, also going across the forum and having 10 "stupid posts" really distracts users so let's try to make post titles more representatives

[Ciro Pabón]

@ mep: yes, your explanation is the same thing I tried to say. You have a low oxygen mixture and you rev the engine using downshift. This is the reason why even "normal" engines with poor ignition some times detonate (produce a loud bang) when you downshift.

@ slick: I also believe that some engines control the rpms by cutting the spark, the same way the gadget I posted works, so this is possibly another reason. We need here an "specialist" to be sure.

@Birel99: well, I always think of an engine as a air pump. The fact that is mixed with gasoline affect little the performance of valves, intake and exhaust. The best engine is the one that pumps air more efficently.

Now, picture for a moment an engine on the first stroke: the cylinder is going down, the intake valve is open and the air-gas mix is entering.

If you close the intake valve just when the piston reaches the bottom of its movement (what is called "BDC" or "Bottom Dead Center"), you are going to stop the inrush of air right when is entering the cylinder at its highest speed.

The air has inertia, like almost anything in this world. So, you left the valve open a little more time, with the final effect that, even while the cylinder is moving up, on the compression stroke, the air is still entering the cylinder because of that inertia, giving you a little extra mixture inside the cylinder.

The opposite is true on the exhaust stroke: even when the piston has moved away from the TDC (top dead center) and it's starting to move down, (theoretically it has started the intake stroke), the air is exiting at top speed and it "pulls" a little extra air out.

Finally, when you overlap the exit and intake of air, the exiting air helps to "pull" in the intake air.

This effect of the "inertia of the air" is more noticeable at high rpms, simply because the air is moving faster. This is why the overlap of the valves is greater in race engines, that develop ultra-high rpms.

The "magic" of valve design and manufacturing resides in closing the damn thing right when the air stops moving across it, not when the cylinder is at an arbitrary "theoretical" position at TDC or BDC. This theoretical position is good only for extremely low rpm engines.

Incidently, this is the reason why it is so hard to keep a racing engine at low rpms (just hear a drag car when it is idle: the engine sound is extremely uneven) and it is so easy to stall a F1 car on the grid or on the pits.

This is also the reason why it is forbidden to have variable timing camshafts, like the hydraulic or electric systems devised for "normal" cars. Actually, I don't understand the reasoning behind it, but if you had variable camshafts, F-1 engines would develop even higher accelerations from a standing start: the engine valve overlapping would be optimum at any engine speed. This is the reason behind the "desmodromic valve" design of some motorcycles.

After writing all this (which for people that work on engines is basic stuff) I am not sure if this is a dumb question. It is important to understand all that if you want to appreciate a racing engine and the kind of compromises you have to make, as a good engineer, between theory and real life.

At least the next time your favorite driver stalls miserably the engine on pits exit, when the race is on the balance, you'll understand that it's not his fault: it is the design behind the engine what makes him miserable. Of course, the best drivers (like, I don't know, JPM or Alonso ) never ever stall the car, but that it's not easy.

Anyway, if you keep saying your questions are dumb, I will be surer that my limited answers in "engrish" can be useful... I only hope that my long posts are clear. You tell me.

[Birel99]

Thanks for all the help.
something that confuses me is the pressure wave that comes back up the exhaust.
it supposedly brings more exhaust gas out, but the return wave is going back into the cylinder?
can someone please explain this whole theory?
so for low RPM's you want as little valve overlap as possible? and the reason that more overlap hurts the low end is because the inertia is not great enouph to get the spent exhaust out and fresh charge in?
Thank you very much,

[Reca]

I wouldn't want to enter into a technical discussion over the other posts, and so wont disagree with the comments made. However, thinking a little more laterally, could the unburnt fuel ignited in the exhaust be down to the engine managment? Under heavy braking and acceleration (Downshift, Upshift) I was under the impression that brake anti-lock and traction control were activated to stabilise the car. Doesn't this mainly involve cutting the spark to the engine? Hence could also be a source for the flames to originate?

Sure, as for example the typical bang you hear when there’s cut of ignition to allow upshift without lifting the throttle. There are plenty of situations leading to unburned fuel ending in the hot headers.

This is the reason behind the "desmodromic valve" design of some motorcycles.

Actually Ciro, desmodromic has nothing to do with variable valve timing, it’s just a mechanical system to close the valve without using the spring. To tell the truth a VVT on a desmo, at least in the Ducati’s form, is probably more complex to do than in a standard valvetrain with springs.

something that confuses me is the pressure wave that comes back up the exhaust.
it supposedly brings more exhaust gas out, but the return wave is going back into the cylinder?
can someone please explain this whole theory?
so for low RPM's you want as little valve overlap as possible? and the reason that more overlap hurts the low end is because the inertia is not great enouph to get the spent exhaust out and fresh charge in?

When a pressure wave reaches an open end (as the exhaust end), the reflected wave that is generated has an opposite sign, meaning that if the original wave was an high pressure wave the reflected one going back is a low pressure wave and viceversa, (when on the contrary the end is closed the reflected wave keeps same sign as original).
So the high pressure wave that starts at the exhaust valves reaches the open end of the exhaust (or for example the 4 in 1, which is still basically an open end), and gets back as low pressure wave.

If the whole game of waves in the exhausts is timed correctly when the exhaust valve opens there’s a low pressure wave arriving there and helping to suck exhausted mixture out of the cylinder. If the thing is timed correctly with intakes too, then this happens during the overlap when, contemporarily, an high pressure wave is arriving at the intake valve pushing more air in the cylinder. This helps to remove all the exhausted mixture in the cylinder filling it with fresh mixture.

Obviously this works only for a particular rpm, depending by length of intakes and exhausts.
When the timing is all wrong you have the worst possible situation, an high pressure wave when exhausts valve opens so pushing back mixture in the cylinder, coupled with a low pressure wave at the intake sucking exhausted mixture back in the intake runners => disaster, you have what we in Italian call “buco di coppia” (literally would be “hole in the torque”) a dramatic drop in volumetric efficiency hence in the torque generated.

To avoid it happening, and to have an engine properly tuned for both low and high rpm you need to change valve timing and/or geometry of intakes and/or exhausts.

Now, to understand why it’s beneficial to reduce overlap at low rpm consider that a given overlap duration in term of crank angle is going to be of different duration in term of time depending by rpm.
On the contrary the other element of the tuning, the time needed for the waves to make the trip from valve to end of pipe and return, is related with speed of sound (hence temperature of the gas) so doesn’t follow the same variation with rpm, ideally, just for sake of simplicity we can consider it constant.

So, what you have is that at high rpm the overlap time is very short and only the useful wave, ideally, arrives in that short interval at the valve.
At low rpm on the contrary the same overlap would last lot of time, in that long time many waves arrives at valves, both high and low pressure waves, some of them are beneficial, some are destructive.
Reducing overlap and opportunely changing valve timing you can select only the good ones.
Changing intake and/or exhaust runners geometry (meaning increasing the length), so that waves going at the same speed will take more time for the complete trip and less waves will arrive at the valves during the overlap, so you can again pick only the good ones.

BTW, this is a simplified explanation of a complex phenomenon.

[Ciro Pabón]

Reca: thank you very much for the clear explanation (and the correction). I remembered vaguely some description (by DaveKillens, maybe?) of a desmodromic AND variable mechanism and somehow both were glued in my mind. As you say, desmodromic valves have nothing to do with variable valve timing: they are positive action valves (you don't have a spring to return the valve, but another cam or similar mechanism).

[Carlos]

A longer intake overlap allows more "fresh" intake charge to enrer the cylinder - a longer exhaust overlap allows more evacuation of "burnt" exhaust. Overlap allows maximum intake or exhaust using the "speed" of intake and exhaust columns "flowing" gases . Tunned intake length takes advantage of a positive pulse to force more fresh charge mixtue into the engine. Tuning the exhaust lenght allows timing a neg or pos pulse to help extract burnt fuel mixture from the cylinder. Overlap takes advantage of the inertia of both intake and exhaust tract columns. I have noticed that header design on F1 engines have begun to resemble the shape and design of 2 stroke expansion chambers.

Post Edited Dec 14th - Editorial withdrawn. Technical opinion has not been edited.

[Carlos]

I know it strays from the post - but I am starting to consider the possibility of not only direct high pressure fuel injection into the 4 stroke cylinders - but also high pressure direct injection of "air" into the cylinder. This technique is already being used in advanced 2 stroke engine research.

Opinions, patents, R&D and PHD thesis citings are welcome.

Another idea is electrostatic rheologic activated valve activation replacing the use of camshafts, using viscosity/volume sensitive polymers activated by electrical pulses.

The ideas that occur between 3>5 AM when I could be sleeping or assisting a companion

[Ciro Pabón]

Carlos: what's the difference between high pressure air injection and turbocharging? I don't get it. Reca, help.

Do you mean to inject the air and fuel mixture at TDC (well, or close to it) on the compression stroke? In that case, you could omit the intake and compression strokes, isn't it? So, how could you use that design on a 4-stroke engine?

If I am following your drift, AND you could overcome the technical difficulties (high probability of detonation?), the engine would be incomparably more efficient that a normal one: you wouldn't waste energy taking air or compressing it (well, I know, compression "energy" would be "expended" when the mixture enters at high pressure, but you get the idea, I imagine).

Right now I guess you should go back to "assisting your companion". This could inspire you even more than these posts!

[manchild]

I think direct MIXTURE injection was patented years ago. If you have 4 valves per cyl. all 4 would become exhaust valves with 2 exhaust manifolds on each side of the engine. Fuel and air mixture injected after exhaust valves are completely closed and than normally ignited. I don't know why that system isn't available yet on passenger cars but I bet it enables extraordinary aspiration. It is logical that some sort of compressor and tank with compressed air must exist to enable staring of engine (probably driven by electric motor).

[Carlos]

Yes - Direct Mixture Injection. I often use posts as a sketchpad for brainstorming. Ciro - Your right, my posts could be more precise and well considered.

Manchild: So it's already been applied to 4 stokes? Fascinating technology.

I'm looking forward to having an original thought oneday I'll aspire to inspiration

[manchild]

I remember seeing a report about it from some car show several years ago. It was mentioned that it is applied on 4 stroke Otto engine but apparently it isn't mass produced yet.

[Mikey_s]

I don't know why that system isn't available yet on passenger cars but I bet it enables extraordinary aspiration

I don't know the answer, but I can think of a few of reasons;

1) thermodynamically the compression has to happen somewhere, so energy will be required to compless the mixture which must be derived from the combustion process. So Assuming that we can't contravene the first law of thermodynamics you can't get something for nothing! A separate compressor would add weight, friction and complexity - probably requiring more energy than the efficiency gains delivered.

2) the safest place to compress a fuel:air mixture is in the cylinder - can you imagine the bang of a compressor tank going up!!

3) volumetrics; it would have to be a large tank as it would need to be large enough to deliver the mixture at the maximum rev limit intake rate, even though an engine seldom operates at that limit, so there would be large scale redundancy.

4) probably there are some octane constraints too, as the compressed fuel:air mixture would have to be significantly above the cylinder pressure to get a timely transfer into the cylinder at high revs.

Sorry to be a party pooper, but I suspect the benefits don't outweigh the efficiency losses.

As an aside, I know that Shell (and probably others) use friction modifiers (effectively lubricants) in the fuel to increase efficiency. The intention was/is to reduce (even by only a tiny amount) the friction between the piston and the cylinder wall on the compression stroke. In this way even tiny reductions in friction multiplied by the number of compression strokes add up to a finite benefit

[Ciro Pabón]

Ciro - Your right, my posts could be more precise and well considered.

No, no, Carlos, your posts are OK! What I meant is that you could spend more time "assisting your companion" in search of inspiration. My wife loved the phrase. Now we don't make love: we "assist each other" (I apologize if I'm interpreting this the wrong way...).

@Mikey_s: good point.