Engine technology thread.

[serialk11r]

Oh okay only one has higher lift, so the swirl effect would help.

Yea I do understand the thermodynamics and that it's a low cost way to get somewhat close to the continuously variable duration system, but nice diagrams I didn't realize that the burn was significantly slower with less charge :O Although I guess that makes sense as without a throttle plate the temperature right before ignition is lower. Maybe variable compression ratio will one day correct this It's also interesting how 90-100 degrees of delay is where everything starts going down. IIRC when compression is effectively 1/2 of expansion in a typical gas engine, the cylinder is at atmospheric pressure by the time exhaust valves open, I can't think of a reason why covariance of IMEP would start to increase at the same time the expansion ratio becomes "too high". Seems like a coincidence of the specific heat of combustion of gasoline :O

Do you know anything about how blowing the charge back out would affect combustion? I imagine blowing the charge back out through a narrower valve opening (which it is by the time the piston starts on its way back up) and then letting it sit in the intake tract until the next intake stroke would be good for making the charge homogeneous, but during the intake stroke itself the cam with higher lift would pose slightly less resistance to flow, so the intake velocity is still lower, seems like a compromise?

[arghx7]

Originally Posted by serialk11r

Do you know anything about how blowing the charge back out would affect combustion? I imagine blowing the charge back out through a narrower valve opening (which it is by the time the piston starts on its way back up) and then letting it sit in the intake tract until the next intake stroke would be good for making the charge homogeneous, but during the intake stroke itself the cam with higher lift would pose slightly less resistance to flow, so the intake velocity is still lower, seems like a compromise?

I don't have a detailed answer to that right now but I have a list of stuff on the Mazda miller cycle engines that I need to go through that might describe the flows and mixture formation better.

[serialk11r]

Oh I have another question if you happen to know the answer, what is the pressure in the cylinder when the exhaust valve typically opens? Of course this depends on compression ratio, but do you have any values you can share? By googling "graph of cylinder pressure in gasoline engine vs crank angle" I see a lot of sketchy graphs that don't look correct (exhaust valve opening event does not cause fast pressure decrease, etc.), but it appears that the pressure is typically in the neighborhood of 3-5 bar (at least for compression ratio around 10). I ask because I wonder how much power you could get from a naturally aspirated engine with a power recovery turbine, which I typed some crap about a lot of pages ago. I saw some figures as low as 1.9 bar, in which case a turbine would recover only a very tiny amount of power. If it's more like 5 bar then the work being blown out the exhaust that can be easily picked up is maybe only 1 order of magnitude smaller.

[arghx7]

Originally Posted by serialk11r

Oh I have another question if you happen to know the answer, what is the pressure in the cylinder when the exhaust valve typically opens? Of course this depends on compression ratio, but do you have any values you can share? By googling "graph of cylinder pressure in gasoline engine vs crank angle" I see a lot of sketchy graphs that don't look correct (exhaust valve opening event does not cause fast pressure decrease, etc.), but it appears that the pressure is typically in the neighborhood of 3-5 bar (at least for compression ratio around 10). I ask because I wonder how much power you could get from a naturally aspirated engine with a power recovery turbine, which I typed some crap about a lot of pages ago. I saw some figures as low as 1.9 bar, in which case a turbine would recover only a very tiny amount of power. If it's more like 5 bar then the work being blown out the exhaust that can be easily picked up is maybe only 1 order of magnitude smaller.

It sounds like you are looking more for information on exhaust port pressure during the blowdown pulse. Blowdown is the exhaust pulse that is generated when the exhaust valve initially opens.



That's a graph created during a Ford comparing exhaust blowdown pulses on I4 engines and 90 degree V8 engines (current 5.0 , BMW twin turbo 4.4, Porsche twin turbo 4.8)

[serialk11r]

I was looking for the pressure inside the cylinder after the burnt charge has completely expanded, to get a theoretical bound on the amount of energy an exhaust turbine can recover. For example if the pressure is 5 bar, then using \gamma=1.3 (I think this is reasonable, since the exhaust gas is mostly small molecules, and the temperature is likely in the range where dry air still has gamma>1.3), under adiabatic expansion we can extract approximately 150J/L. If this is at 7000rpm, 2L engine, this comes out to 17.5kW (23.something hp)! This is inline with the observation that the Prius engine with higher expansion ratio has around 10% higher peak thermal efficiency than an Otto cycle engine (which lose efficiency at full throttle due to fuel enrichment), despite its low dynamic compression of 8:1 under WOT. Of course the exhaust valves themselves create restriction and the turbine would be mostly working off the transfer of momentum of the gas which isn't so good in a piston engine with poppet valves in the first place, so the actual amount of power that can be recovered is likely much lower, but even if it's 10hp, that's not bad for just a turbine bolted to the exhaust.

[arghx7]

look at the indicator diagrams and heat release diagrams in the papers in the sticky thread

[serialk11r]

Thanks.
Using my eyeballometer with a lot of zoom in it looks like ~3-4 bar. So I guess exhaust pressure recovery is only effective with forced induction...
EDIT: actually at higher rpm the fuel mix burns slower relative to crank speed, so the effective expansion ratio drops and things might be a bit different...that chart was for 3600rpm WOT operation. At 7000+ things are probably pretty different. Hmmm.

[serialk11r]

Hey guys so you know how the FA20 has a relatively "hot" cam given its torque peak at 6600? I never realized that closure of the intake valve happened so late on high rpm cams (if I am reading correctly, up to 90 degrees after BDC? O_O). That reminded me of dual VVT-i, if the cam already had a long duration, then VVT-i could be pretty useful for pumping loss reduction, as Toyota claims. I was always under the impression that by playing around with the cams they could only reduce pumping loss by a tiny bit. I found this, and I'll ramble for a bit on how I think a continuously variable duration system could improve on it:

http://www.mr2.com/files/mr2/techinf...a/3grfse-2.pdf
Page 66 shows how they have the cam retard/advance set up. Low load cam looks to be early exhaust opening (good, at idle the expansion ratio is too high), intake valve closes about 70 degrees after BDC, so about 1/3 of the charge is pushed out of the cylinder. The exhaust valve opens about 30 degrees earlier than normal, so the effective expansion ratio looks like about 8:1. Assuming they don't overexpand the burnt charge, the compression would need to be somewhere around 5:1 (very rough). So if 1/3 of the charge is pushed back out then the throttle only needs to pull 1/4 bar vacuum (at very light load, at idle it would be a bit more to consume the power). Pretty good, would be even better with say, a 260 degree intake cam. Valvematic could reduce the loss at the throttle a little bit more, but not much.

At medium load is a bit curious though IMO. They give it the same overlap as high load to increase EGR, to reduce emissions. I believe Toyota says they have a maximum internal EGR rate of 20%, which I hopefully am interpreting correctly as 20% of exhaust gas volume stays for the next cycle. Looking at the intake, it appears to be closing at the point of highest volumetric efficiency... hold on a minute this doesn't sound so good for fuel economy. EGR gases at high temperature reduce the heat capacity ratio of the charge, slow combustion, and non-delayed intake causes more pumping loss. Must be an emissions reduction technique I believe if the intake opens a bit later, it closes a bit later, and pushes some charge out. This reduces EGR a bit which improves efficiency, although a greater proportion of the charge is fresh air so pumping may not be reduced. The newer engines and in particular D4-S seem to have improved on emissions, it is possible that they shifted strategies. Valvematic would be able to eliminate the pumping loss and the EGR (if desired), seems to be a good efficiency boost.

The next part is perhaps the one that most people care about the most, and I think it explains the drop in mid range torque. For high load, low-medium speed, the long duration intake cam needs to be advanced to stop charge from blowing back out. But EGR was already very high before the cam was advanced! It seems that the intake valve opens early and allows fresh exhaust to blow into the intake D: This would certainly hurt volumetric efficiency quite a bit. I'm guessing that they would rather go with high EGR than delayed intake + low EGR which would probably produce similar power but slightly higher emissions due to lower combustion temperature. Here Valvematic would allow highest possible volumetric efficiency. The low speed "torque peak" due to improved combustion efficiency via D4-S on the 2GR-FSE can't quite hit the absolute maximum torque because it has too much EGR and some amount of late intake valve closure. I am guessing that EGR reduction could improve efficiency here because EGR could possibly cause not so ideal ignition timing due to knock, and the fact that efficiency is higher with a lower effective compression and the same effective expansion.

I'm not too sure how important scavenging is but it appears that at high speed, high load, the overlap is gone so scavenging can't really happen. In addition, it's worth noting that the exhaust opens rather early (I think? big maybe here...can anyone help me on this?), and I can't help but think that this is a compromise made for lowering idle consumption, although I may be wrong. I am guessing that Valvematic could enable a slightly shorter duration exhaust cam to cut EGR and at high speed, increase the effective expansion, as the lower lift settings improve fuel atomization.

Related: Taking note of the fact that D4-S engines seem to use a high speed optimized cam with long duration, and still have VERY good peak BSFC, as I said before it appears D4-S can create good combustion efficiency even with low intake velocity. I predict that longer duration cams could probably increase fuel economy.

Anything wrong? Any ideas?

[Dimman]

Nice find.

Is there a lift ratio on these rocker arms, like the domestics? (1.5:1 for example) Because 5.99mm cam lift for 34.5mm intake valves is TINY. ~8.5mm lift puts it at ~0.25 dia.

[arghx7]

They look pretty close to the generic diagrams for the regular 2GR-FE engine with port injection (see attached). Without having the exact VVT maps it's hard to draw too much conclusion. Lots of Subaru maps VVT/AVCS maps are floating around for the 08+ STi which has variable valve timing on both intake and exhaust. For the most part it follows the patterns described in the Toyota manuals. It's the same for say an Evo X.

You have zero overlap at idle, significant overlap in the mid range, and minimal overlap at high rpm depending on the dynamic effects involved.

[serialk11r]

arghx7, it appears to me that the exhaust valve at high power opens rather early, do you think there could be peak power gains from shortening exhaust duration? Not 100% sure but it feels like the long exhaust duration is well suited for low-medium load efficiency and low-mid rpm, high load emissions control, but under heavy load and late intake closure, it doesn't look to be ideal. This is in line with the typical priorities of the OEM, emissions and low speed efficiency.

EDIT: I'm basically trying to figure out if a "racing cam" (not actually necessary, but I don't mind crappy low end torque) plus modified VVT-i maps can improve fuel economy, and maybe fill in that mid range torque gap a little, since something tells me Valvematic won't make it to this car...current engines are hitting around 35% thermal efficiency, friction reductions can't trim much more, exhaust heat recovery isn't here yet, so peak efficiency is basically near its limit, and there's not any easy gain. Automakers need to hit like 35?mpg by 2016, which they don't need variable duration to accomplish. By 2025 that needs to be 50mpg or so, but can't wait till 2025 to get a car

It appears that D4-S doesn't seem to mind high lift (would anyone happen to know whether it is usually duration or flow that limits higher rpm power?), which is awesome. A slightly retarded long duration cam could give stellar low rpm fuel economy, as low load efficiency is greatly affected by pumping loss. Cutting charge density by just a little would give a nice bump in low load efficiency. I think it is possible to help our wallets out a bit by straying a little from the EPA's guidelines...besides don't automakers try to aim for the lowest emissions standards? Slightly higher emissions could still pass. It's not like this is changing AFR, this is just reducing EGR.

[Homemade WRX]

^^you hit the nail on the head!

I have a saying...
You have clean (emissions), powerful and fuel efficient; pick two.

From the factory with scrupulous EPA, CARB and Euro mandated emissions tests, you have to make sacrifices. This is why, as you commented on, the targeting of low to mid load and RPM in the camshaft.

Wasteheat covering is actually make leaps on and bounds in research in the industry, with several methods being testing currently.

[serialk11r]

Is it? Last I checked thermoelectric generators still had a horrible Z and high temperature ones have slightly better ZT but the high temperature implies more energy to be extracted anyways. I've read papers on various mechanical engines for heat recovery, I'm not sure how promising they are. Shape memory alloy engine claim to have good power to weight, except that they have inherently low efficiency, that TEG could exceed with better Z values (which they are promising)...I saw a paper that described an SMA engine that seems to get rid of hysteresis losses and frictional losses by putting the wires into a chain with solid linkages. As for steam, engine coolant is typically near boiling point, but boiling ethylene glycol and feeding it through a turbine might not be the best idea...I dunno. Heat exchangers take up a lot of space and weight unfortunately, and there is no avoiding them with a steam turbine.

And as we all know, the time it takes for stuff to get from the lab into our cars is quite a while It's funny cuz I'm doing math (well not quite the legit research level yet) and that stuff takes hundreds of years to make it into the real world. I heard the SMA thing is being developed by GM, hopefully a lot of Uncle Sam's money can speed things up a bit :P All the generator really needs to do is power the electrical system at low speed cruise power, which at 45mph is like 15hp. When the engine puts out 15hp there is about 15hp in the exhaust in heat alone, and at 10% efficiency (exhaust temp around like, 700C at part load? let's say max Carnot efficiency is 60%) that is like 1kW. So nothing lightweight can hit 10% efficiency yet -_-

EDIT: I just looked at BMW Turbosteamer again, they claim 10% efficiency gain with single loop Rankine cycle. This drops to under 8% at full load. So the system needs to weigh less than 8% of the car's mass to be worth the extra power...200 pounds is not too bad of a mass budget. I guess it has some hope, we'll see.

[arghx7]

If you put in more aggressive cams in terms of lift and duration, you could probably find a way to make it work. There isn't a whole lot else to say without more specs on the actual engine.

Having continuosly variable cam phasers does reduce the marginal performance benefit of performance cams. There's less to be gained from spending the money.