Compression ratio vs engine speed?
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URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=102825
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Topic: Compression ratio vs engine speed?
Posted By: AaronSEIA
Subject: Compression ratio vs engine speed?
Date Posted: 05 Mar 2015 at 7:20pm
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So the thread on the AC page over at YTMag is trying to blow up over theoretical CR vs engine RPM. One guy is saying that in a stock WD45 or D17 that 7.25:1 is it without running special gas or soemthing due to the low rated engine speed. The 170 is 8:1 with a rated speed of 1650. Dr Allis and others say that 8:1 is fine. I know this is a pulling page, but how do CR and engine RPM relate? Is there theoretical max ratios for given engine speeds? AaronSEIA |
Replies:
Posted By: DrAllis
Date Posted: 05 Mar 2015 at 7:44pm
| When a One-Seventy was tested at Nebraska test, they lugged it down to what is called "peak torque". That is the RPM where the maximum foot pounds is achieved. Where does an 8.0 to 1 One-Seventy reach peak torque you ask ???....800 RPM. I can assure you with 87 octane gasoline there was no "pinging" at Nebraska Test. If the ignition timing was set faster than 25 degrees BTDC, you can bet it would ping, but not when properly tuned. A stock D-17 was 900 RPM if I remember right. |
Posted By: wi50
Date Posted: 07 Mar 2015 at 7:42pm
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maybe i'm wasteing my time, but its the pulling forum so I'll give a brief explanation of compression ratio related to speed. Some say compression is king, well they haven't thought it through very far. Compression is good, the higher the cylinder pressure is, the better the fuel mixture burns. The higher the pressure is in a cylinder, the faster the flame will travel across the bore. This is why a higher compression engine would need less ignition timing than a lower compression engine with all other things equill. But a higher compression ratio has a faster expansion rate, meaning that when the cylinder fires and the gasses expand, the piston moves down the bore the piston is moving at a higher rate, or the bore volume is increasing faster than in a lower compression engine. So we fire the engine at say 25* BTDC, the mixture starts to burn and it's still getting compressed. Piston hits TDC and starts down the bore, the high pressure and rapidly burning mixture pushes the piston down. The effective stroke length of the crankshaft isn't much though for the first few degrees of crank rotation, At 90* of crank rotation for example a 6" stroke has a 3" leaver, or there's 3" of offset from the center of the crankpin to the center of the crankshaft. But that mixture is done burning long before 90* ATDC or down in the power stroke. A high compression ratio releases it's energy sooner and more violently than a lower compression ratio can. And the leaver or stroke length at TDC is basicly 0. So this high compression engine releases it's energy early in the power stroke, and it releases it faster and the cylinder volume expands faster as the piston comes down. The lower compression ratio will "push" with less force but push for more degrees of crankshaft rotation than the higher ratio. And the cylinder expands at a slower rate. Now lets look at the fuel burn rate. It burns faster in a higher pressure than in a lower pressure. But more energy is released in a higher pressure cylinder. So we need compression to get the energy released from the fuel. In a high RPM engine we have less time to burn than in a lower RPM engine. So a lower RPM engine would need higher compression to get the fuel burned in the same time as a higher RPM engine with all other things being equill, Now we hear that more compression is better, jack the compression ratio up. Well higher compression will release the available energy faster and it's great for an engine running at a higher speed. In the antique tractor engines though we don't have much speed, RPM is low and our milliseconds to burn that mixture are longer. A lower compression ratio will allow the "push" to continue for more time, but we still need more compression to get the cylinder pressure high enough to release that available energy. A 14:1 compression ratio may be fine for an engine running 7000 RPM but it will destroy an engine running 1500 RPM. The only way that some can get away with it in the antique tractor engines is because the induction system is so poor, the cylinders are just starved for air and not nearly as full as in an engine with a good induction system. So if we built an engine that has to run low speed and starve it for air, we can get by with say a 12;1 compression ratio and run it slow. But if we can figure out how to feed it efficeintly we can build it with a 10;1 ratio and have a better performing engine that is easier on it's parts, and the 10;1 will lug longer. I run methanol in an engine, that doesn't quit burning. It's pushing on that piston a long time and that engine will not go to sleep. The power stroke is long compared to a faster burning fuel. It's also very easy on the engine parts. A stock crankshaft and light alumnium rods hold up fine. Even the hardware holding the cylinder head on is nothing special. It's got lower cylinder pressure than a diesel and pushes longer on the crank, that's how we make 3500 horsepower from a 505 cid engine. The top fuel cars burning nitro can run less boost and less cylinder pressure and make over double the horsepower because nitro just doesn't stop pushing on that piston untill the exhaust valve is pushed into a high pressure cylinder. ------------- "see what happens when you have no practical experience doing something...... you end up playing with calculators and looking stupid on the internet" |
Posted By: AaronSEIA
Date Posted: 08 Mar 2015 at 5:07pm
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Until sones disagrees, that is an excellent explination. Clears up a lot of questions in my mind. The big question from this is...in a stock WD45 or D17, does a 7:1 vs 8:1 ratio make any difference. I'm sure something can be gained with 87 octane going from the original 5.5 or 6.5:1 up to 7.25:1. Is anything gained or lost or longevity going to 8:1 or higher for a straight up parade and plow day machine? AaronSEIA
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Posted By: HudCo
Date Posted: 08 Mar 2015 at 6:59pm
| yes that is a good explanation low octane gas is designed lower compressoin engines try this take your socks off soak one in regular unleaded and the other in race gas the one in regular will go up and be gone before the match gets close the one in race gas will take a box of matches to get burned up |
Posted By: DrAllis
Date Posted: 09 Mar 2015 at 4:50pm
| 8.0 to 1 makes more HP than anything less on the same 87 octane gas. That is a fact. It will also be more fuel efficient, which may not be a big deal, but it will use less fuel on the same job as tractors with less compression. The other thing is, most every one of those after-market 4 1/8" kits are lower compression AND bigger bore so fuel use will go up for sure. |
Posted By: WF owner
Date Posted: 09 Mar 2015 at 6:56pm
| Doc, What pistons give you 8.0 to 1 compression with a 4.5" stroke (226) engine? |
Posted By: DrAllis
Date Posted: 09 Mar 2015 at 9:30pm
| One-Seventy are 8.0 to 1.......175 is 8.25 to 1.....both 4 inch bore. |
Posted By: mgburchard
Date Posted: 10 Mar 2015 at 8:59am
| Doc a is right and Allis Chalmers is right the higher the compression the lower the rpm peek torque will be even with the largest factory cam as allis did with the 175 cam. Higher torque and hp comes from compression wither it's from piston blower or turbo top fuel runs 65 psi of boost per professor Pankey |
Posted By: Hudsonator
Date Posted: 10 Mar 2015 at 9:29am
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I always looked at the situation from a feet per minute (fpm) or feet per second (fps) in relation to the burn rate and expansion front. I didn't want the speed of the piston to be "outrunning" the pressure wave of the burn. Its entirely possible to "outrun" your pressure wave near the bottom of the stroke and create a "negative" pressure that offsets some of the positive pressure of the burn. Slight? Yes. But any decrease in the force applied to the "leverage" Marty outlined, is a loss - plain and simple. However, your fpm has to begin to exceed the rate of burn/expansion to see this loss. Its also possible to "under-run" your pressure wave - which is what Marty explained. Longer stroke engines have a faster fpm than a shorter stroke engine at the same rpm. WD 4" stroke vs WD45 4.5" stroke is classic. FPM = 2(stroke in feet x rpm) 4" stroke @ 1650 rpm = 1100 fpm 4.5" stroke @ 1650 = 1237.5 fpm Now, lets compare a 6" @ 1650 = 1650 fpm I'm not in any way arguing with Marty, just enhancing his contribution. This is why excessive high compression and efficient induction in a low speed engine kills it - the low piston speed and inadequate cylinder volume per force basicly creates a pipe bomb in the cylinder. When the force exceeds the ability of the rod, rod journal, and/or piston to contain it - breakage occurs. I've seen a bigger problem in antique tractor engines from the "pipe bomb" effect than I have a loss of power due to compression. In high rpm engines, I've seen the opposite or "outrunning" effect put a cap on Hp because the piston speed is moving faster than the pressure wave. ------------- There isn't much a WC can't do. WD's just do it better. |
Posted By: mgburchard
Date Posted: 10 Mar 2015 at 9:43am
| Yep got to have good parts |
Posted By: mgburchard
Date Posted: 10 Mar 2015 at 9:45am
| Yep got to have good parts |
Posted By: mgburchard
Date Posted: 10 Mar 2015 at 10:08am
| All this time fooling with four cylinder engines I thought one piston was being pushed by the pressure wave of the explosion during the power stroke for the other pistons on intake compression and exhaust stroke to reach 1650 fpm. Also a six inch strokes pistons speed is identical to the rpm |
Posted By: mgburchard
Date Posted: 10 Mar 2015 at 1:47pm
| Dr a runs them at 13 /1 read his post on y t |
Posted By: mgburchard
Date Posted: 10 Mar 2015 at 1:49pm
| Mitch is the MAN |
Posted By: blue924.9
Date Posted: 10 Mar 2015 at 2:02pm
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dont do it boys, i checked his profile he is sittin watching this topic waiting for someone to take the bait. ------------- hi my names dan, I am a young guy. i have a problem, i prefer my tractors orange and my clutches mechanical, thanks for letting me share |
Posted By: mgburchard
Date Posted: 11 Mar 2015 at 12:25pm
| You are learning blue when someones wrong and doc a and Mitch are right |
Posted By: wi50
Date Posted: 27 Mar 2015 at 12:51pm
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I've just been to busy to spend much time on here, but Hudsonator is exactly right. It's easy to get the pressure wave from combustion to outrun the piston. Or in a sense set off a bomb in the cylinder compared to a push. Next is the expansion ratio, the higher compression ratio has a higher expansion ratio so it dumps it's pressure faster, that is good....but not in the case of a low rpm engine or one that needs to make big torque numbers at low speed. In this case we want to push on the crankshaft a bit longer, the longer we push, the more effective stroke length gets used. Or if I dumb it down for a certain crowd, "the leaver gets longer". We have a certain amount of time to push on the piston, in a 4" stroke engine that makes great power at just say 5000 RPM we have a mean effective piston speed of say 3333 feet per minute. We have a verry brief amount of combustion time pushing on the piston. The "bomb" can explode and be contained while being effective. Lets apply that same time to a 6" stroke engine, like we may build for the tractor pulling engine. We would have to run that engine 3333 RPM in order to get the same time / pressure applied, except for the connecting rod to stroke ratio plays a little into this, as the poorer the ratio, the quicker the piston will acclerate. Again it will have negative effects and positive effects. The people who will sit back and preach that more compression is better are not the people out there doing well and winning. They are the people who are still trying to figure things out, some of them do and some of them never will. If I have an engine that breaths well and build it to 10:1 compression ratio, I will have more cylinder pressure upon firing than a poor breathing 14:1 engine will. The 10:1 has a slower expansion ratio and applies pressure to the crank longer than the 14:1 can. If I were to build it at 8:1, it may be to weak, simply not enough compression to do an effective job, apply some boost though and see what happens. The same people that claim compression is king, are not getting their induction system right. They are compensating for lack of air with excessive compression to try and get enough cylinder pressure to do the job, but the higher expansion rate and the inability of the piston to outrun the pressure wave just make for a bad combination. The engines beat themselves up, don't last and really don't make any impressive power, the power band is also very narrow in comparison to one that is designed and built right. ------------- "see what happens when you have no practical experience doing something...... you end up playing with calculators and looking stupid on the internet" |
Posted By: Hudsonator
Date Posted: 28 Mar 2015 at 9:03am
Have you worked out a general "rule of thumb" in regards to expansion rate and piston speed? I'm one of those who has used increases in compression to compensate for an induction system that just could not be modified to approach anything efficient without "forcing" the issue - which wasn't an option. When there is nothing else left to do, it works - but has a practical limit. It does narrow everything: timing curve, fuel curve, and resulting power band. ------------- There isn't much a WC can't do. WD's just do it better. |
wi50 wrote:Posted By: mgburchard
Date Posted: 28 Mar 2015 at 5:37pm
| Be good if you had proof to back it up . The facts are at the mid Atlantic pull in pa last week .High compression engines won most the classes even in three mile an hour with a little high compression case build. To end this discussion Taylor and Taylor published a internal combustion book . That says as engine rpms increase flame propagation increase proportionaly Said this is why a spark advance can be all in at a low rpm and still be correct for 7000 rpm |
Posted By: wi50
Date Posted: 29 Mar 2015 at 8:37am
That would depend on your definition of "high compression". But speaking of the pull results, be sure to congratulate yourself and crew on last and near last place finishes. When I checked the results it looks like the guys I work with put 30-70 feet on team spankey. I always thought the objective was to pull the furthest. ------------- "see what happens when you have no practical experience doing something...... you end up playing with calculators and looking stupid on the internet" |
Posted By: wi50
Date Posted: 29 Mar 2015 at 8:44am
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And the pankey sags continues. Don't worry folks when he's done licking his wounds he'll be back with more make believe and science fiction. Anyways hudsonator. I'll add a little compression to the big engines to make up for their inability to fill the cylinder if they are limited by rules on certain components. But a big empty cylinder coupled with a faster expanding cylinder only result in an engine with a narrow power band. Generally they run the same gear ratio, and of course a better managed engine will maintain more rpm when both are already starved for air. 9 times out of 10 you are better off with a little smaller and better managed engine......and the results pankey is referring to show it. I'll give up 10% on engine size for the ability to run better rod to stroke ratio and fill the cylinders to a higher %. ------------- "see what happens when you have no practical experience doing something...... you end up playing with calculators and looking stupid on the internet" |
Posted By: mgburchard
Date Posted: 29 Mar 2015 at 9:07am
| Your guy wi50 came over wanting help tuning his engine . i believe it was the engine out of your WC. Your guy cotton crazy wants a larger engine after performance at tunica . The high winding old WC engine 2100 rpms stayed in mile an hour classes didn't 15 mile an hour . Roger Ellwood's 420 inch WC runs good doesn't it. |
Posted By: AC200Puller
Date Posted: 29 Mar 2015 at 9:43am
| Lots of good information Wi50 |
Posted By: mgburchard
Date Posted: 29 Mar 2015 at 12:03pm
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Posted By: wi50
Date Posted: 29 Mar 2015 at 3:37pm
I see. Last week my old WC engine was in a D-17 out there you said, you were looking for video of it. Now there was only one D-17 at the pull and it had a D-17 engine in it, that's the truth. That guy is running a stock cylinder head, stock manifold and carb on an engine I sold him a few parts for. I see he managed to put 30 feet on your crew.....that has to bee a big kick in the nuts. All pankey's knowledge and someone with mostly a stock engine beats up on it by 30 feet. It's not suprising, but it is funny. Today you say you were supposed to help tune it. So this must mean that you seen it..... are you unable to tell the difference between a D-17 and a WC? It appears so. You're a bit on the slow side to catch on here (as always). How would a fella run an engine like that at low RPM and in a 3 mph paced class? I'm just curious, as it would be useless. It would work well however in the 15 mph class. After you've been collecting pictures of it for 5 years and obsessing over it, you must be bright enough to realize that it would not even light at a low RPM or speed....... The lies continue, I see you have not learned anything. But I'll let you in on the truth. That engine is here in the tractor. But it's nice to know you drove 9 hrs to watch a pull. You avoided disapointment by watching and not participating. That I'll give you credit for. I hope you got a chance to stop by and check out my cylinder head on display at the mid atlantic super pull. Friends of mine were there with one on display and talking to people. Lawrence and Richard mentioned they were approached by some guys with an AC that were not happy with it's performance. I think they said something about Ac nightmare or morison or something to that order. I see the results from the pull confirm a lack of performance. Lawrence is pretty sharp with engines though, I'm sure he can help them out. If not I've got cylinder heads, camshafts, etc. on hand and can ship out shortly. ------------- "see what happens when you have no practical experience doing something...... you end up playing with calculators and looking stupid on the internet" |
Posted By: mgburchard
Date Posted: 29 Mar 2015 at 4:19pm
| Everyone seemed happy with Pankeys stuff . The guy with WiFi stuff wanted some tips on timing his .Lawrence would have traveled further than us . Heres a question what mountain would the tractor have to roll down in road gear for the piston to out run the flame propogetion. I thought a pulling engine pulled on a flat track dragging the sled behind it and its flame propagation drove the piston down turning the crank to what ever rpm it can reach due to combustion .isn't that how internal combustion engines works. So what external force let's the piston out run the flame and wouldn't that external force be what's doing the work so the external force would be defined as the motor.Maybe in Wisconsin the motorized sled pushes the tractor making the piston over run its flame propagation. Lol |
Posted By: mgburchard
Date Posted: 29 Mar 2015 at 4:47pm
| Roger with the 420 chic inch WC had the biggest engine and biggest tire and walked away with the wins no flame propagation problems for him |
Posted By: wi50
Date Posted: 29 Mar 2015 at 5:31pm
You need some geography lessons too? You have it backwards. More lies. I wouldn't be happy with last place and next to last place finishes, but if you say they're happy, then I suppose they're happy. Happy it didn't blow up, but that's about it. Anyways go back and read what Hudsonator and I are trying to explain, you seam to have that backwards also. I'll dumb it down to a level I hope you can comprehend. Problems occur when the piston doesn't get away from the "bomb" ------------- "see what happens when you have no practical experience doing something...... you end up playing with calculators and looking stupid on the internet" |
Posted By: mgburchard
Date Posted: 29 Mar 2015 at 5:50pm
| Oh so its a problem that occurs in.long rod engines that have increased dwell at tdc . I see never had that problem with the big inch engines rods are short. The high rving engine finished better in the 15 mile an hour classes . The big 420 inch won the class. No aluminumed up d17 in those classes but I see why the guy needs help timing it now |
Posted By: ACFarmer
Date Posted: 29 Mar 2015 at 7:44pm
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Just stop feeding the troll wi50. Maybe he will go away.... ------------- Making A living everyday farming with and working on Allis Equipment |
Posted By: wi50
Date Posted: 29 Mar 2015 at 9:12pm
I know ac farmer. But don't you want to see what he will make up next? Now it's "aluminumed up" d17 and the guy needs help timing it...... well I said before there was only 1 d17 at the pull....and it certainly wasn't "alumed up". It did however beat on pankeys boys pretty bad, and it's got a bunch of bone stock parts....our troll never even went to the pull, he's just lying. Good for Sunday entertainment, but not much use for any technical help. I ------------- "see what happens when you have no practical experience doing something...... you end up playing with calculators and looking stupid on the internet" |
Posted By: Butch(OH)
Date Posted: 30 Mar 2015 at 8:41am
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For those with curiosity a link to the results. As usual the facts came pretty straight from Marty and Pank is FOS for the most part. Pank's super world beater puller from bragging post last fall is 100' out of first place in one class and 5th out of 6 in the other. This is not the performance levels we read about in your posts Pank??? Whats happening here?
http://midatlanticantiquetractorsuperpull.blogspot.com/" rel="nofollow - http://midatlanticantiquetractorsuperpull.blogspot.com/ |
Posted By: Hudsonator
Date Posted: 30 Mar 2015 at 9:50am
Rod ratio is the most abused, overlooked, yet critical elements to effectively using the rest of one's numbers in the CID equation. I also default to lower cubes when either livability or effectiveness start to go away. Big numbers are one thing, making them work effectively is another. Interesting stuff. Spent a long evening at cotncrazy's yesterday picking up parts for the working fleet. Talking to him and his Dad made me remember how much I enjoyed pulling, and why. ------------- There isn't much a WC can't do. WD's just do it better. |
Posted By: mgburchard
Date Posted: 30 Mar 2015 at 12:11pm
| Compression makes power up to 17.1 as general motors engineering studies show . makes peak torque at lower rpm and hp across entire curve . Taylor and Taylor says in their book that flame propagation and rpm are proportional . Timing one by ear and looking at ignition timing with light to see where it is gets this topic squashed nothing really to it that correct ignition timing doesn't cure . This is also as Taylor wrote why ignition timing can be all in and even locked at a low rpm and still be correct for higher rpm . Hudsonater was the one if WiFi goes back to his post and reads that said piston could out run flame propogation which is not right with correct ignition timing . Taylor also said in his book that to low a compression at a high rpm imparts severe inertia and kinetic energy on the rod as it is going from btdc to tdc resulting in lower half rod bearing damage from the lack of compression canceling out the inertia and allowing top halve of rod to move away from crank journal the amount of clearance it was originally built with and increasing clearance with each stroke as rod bolts stretch or rod halve eggs . In Taylors book there is a formula to figure the connecting rods inertia and correct this issue by knowing you have enough compression to cancel out the rods inertia . Great book but does shed light on who's fos as butch says . |
Posted By: mgburchard
Date Posted: 30 Mar 2015 at 12:25pm
| Butch watch the YouTube videos blowing the tires off is not a sign of lack of HP or poorly tuned engine. |
Posted By: wi50
Date Posted: 30 Mar 2015 at 7:40pm
I could swear you have less than a basic 5th grade education. Now go show us where I said that......you are making things up again. However, I did make the statement that you don't want the "bomb to outrun the piston" which if I dumb it down for you would mean.....well I can't dumb it down enough, you'll only get it backwards. It's nice to know though that we can now build all engines at 17 to 1 and don't need any ignition advance or retard. Do it to prove us wrong and let us know how well it works for you. ------------- "see what happens when you have no practical experience doing something...... you end up playing with calculators and looking stupid on the internet" |
Posted By: wi50
Date Posted: 30 Mar 2015 at 7:57pm
I'll make it easy for ya pank. This is what you're hung up on, and you have it backwards when you say that I say the piston can outrun the flame.....which doesn't happen. What I'm explaining here is on the order of detonation.... you know that phenomenon that destroyed your engine. ------------- "see what happens when you have no practical experience doing something...... you end up playing with calculators and looking stupid on the internet" |
Posted By: mgburchard
Date Posted: 31 Mar 2015 at 7:34am
| Pankey 2010 engine shown no signs of detonation something I guess you've had problems with and are hung up on and why your guys can't seam to get a handle on ignition timing . |