SpitFire6 Posted July 10, 2019 Report Share Posted July 10, 2019 "The main reason for the size difference is to avoid pre-ignition and knocking. The exhaust valve is the hottest part of the engine. We also know that Q=m Cv dT. Hence the heat generated is directly proportional to the mass of the substance. Now if the size of the exhaust valve increases, its mass increases. So the heat of the valve increases. Now after the exhaust stroke, the charge(air and fuel) is sucked in for the next cycle. If the temperature of the exhaust is very high it may cause pre ignition. Also even if the temperature of the valve is not that high, during compression the charge becomes more volatile. The exhaust valve will ignite the charge creating a flame front which progresses in all directions. Also the spark plug creates a flame front. When these two flame fronts meet, knocking takes place. To avoid these difficulties the exhaust valve size is kept smaller than the inlet valve." ............From Quora. It's been +voted 80 times. Bless the internet for information. Now while sipping a pint, with your mates (Gender fluid), you can show your IC engine knowledge by repeating the above verbatim Add your upvote on Quora & change the facts. Or is the joke on me? Cheers, Iain. 1 Link to comment Share on other sites More sharing options...
NonMember Posted July 10, 2019 Report Share Posted July 10, 2019 This is the problem with the Interwebz. It's full of sites that follow the current politically correct trend of valuing information by its popularity, not its accuracy. Ignorant people spout their ill-considered theories and other ignorant people up-vote them because they sound plausible. Or at least, comfortable. Or more cheerful than those doom-sayer so-called "experts" who've only studied facts for many years and keep telling us things aren't as happy-happy as we want them to be. Ah, sounds like certain other topics not to be discussed here. 🙁 I'm sure you know the real reason for valve sizes, Iain? 1 Link to comment Share on other sites More sharing options...
JohnD Posted July 11, 2019 Report Share Posted July 11, 2019 Well, go on, NM, don't leave us in ignorance! J. Link to comment Share on other sites More sharing options...
johny Posted July 11, 2019 Report Share Posted July 11, 2019 I always thought that as its more difficult to suck in air through a hole than blow it out the latter can be smaller and still be sufficient.... Link to comment Share on other sites More sharing options...
NonMember Posted July 11, 2019 Report Share Posted July 11, 2019 Yes, as johny says. The mass of gas* going out is the same as that coming in, but it's got a LOT more pressure behind it. There's no benefit to bigger exhaust valves and they would just take up valuable space that you could use to fit bigger inlets instead. Also, higher exhaust gas velocity from not over-sizing the valves gives a small benefit in gas scavenging (which helps draw in more fresh charge) and swirl (for better mix and combustion). * Well, technically, the mass of the exhaust gas is more because the fuel in the intake charge isn't all in gas form, but... Link to comment Share on other sites More sharing options...
johny Posted July 11, 2019 Report Share Posted July 11, 2019 Course there's nothing to say couldnt be more than one reason..... Link to comment Share on other sites More sharing options...
Pete Lewis Posted July 11, 2019 Report Share Posted July 11, 2019 i see Johny is back to all the sucking and blowing, wrong forum ............again cant blame Doug he's in sunny scotland ......... mother sitting is this why breaking wind takes more energy that a deep breath ???? Pete 1 Link to comment Share on other sites More sharing options...
Colin Lindsay Posted July 11, 2019 Report Share Posted July 11, 2019 33 minutes ago, Pete Lewis said: is this why breaking wind takes more energy that a deep breath ???? Pete Nature had it right all along; most (most!) people's inlets are larger than their exhausts. 1 Link to comment Share on other sites More sharing options...
poppyman Posted July 11, 2019 Report Share Posted July 11, 2019 1 hour ago, Colin Lindsay said: Nature had it right all along; most (most!) people's inlets are larger than their exhausts. I dont know about that Colin..... A few people talk through their exhausts sadly.........Usually those with a lot to say. Tony. 1 Link to comment Share on other sites More sharing options...
Colin Lindsay Posted July 11, 2019 Report Share Posted July 11, 2019 Usually those with leaded heads..... Link to comment Share on other sites More sharing options...
poppyman Posted July 11, 2019 Report Share Posted July 11, 2019 Got it in one mate..... Tony. 1 Link to comment Share on other sites More sharing options...
JohnD Posted July 11, 2019 Report Share Posted July 11, 2019 But comedy aside, couldn't agree more with NM! And, a valve loses its heat via contact with the valve seat. A larger valve will have more contact area. As the valve diameter is raised x2, so the area is increased by 2Pi. The volume increases by only 2. John Link to comment Share on other sites More sharing options...
rlubikey Posted July 11, 2019 Report Share Posted July 11, 2019 Hold on John. Circumference is Pi*d but area (hence volume of the penny on the shaft, assuming thickness is constant) is Pi*r^2. There's a power of 2 on the area. So a larger valve will have less contact area in proportion to the volume, assuming the thickness is the same. There is indeed some sense in the smaller exhaust valve running cooler theory. Or is that what you and NM are saying? It's been a long evening in the pub! Cheers, Richard Link to comment Share on other sites More sharing options...
NonMember Posted July 12, 2019 Report Share Posted July 12, 2019 I think John skipped a bit on explaining the area. The reason four-valve heads work is that the space available for valves is mostly governed by face area but the area available for the gasses to flow is circumference times lift. Four small valves has a much higher flow area to head area ratio than two big ones. As to running cooler... it's very marginal. The reason an exhaust valve gets hotter than the rest of the combustion chamber is that the hot gasses flow past it but, as I've just said, this depends on the circumference, not the area. As its main means of losing heat is the same surface that provides its main means of gaining heat, they scale in proportion. I haven't read any studies on whether valve size has much effect on valve temperature. Given that I've worked in the motor industry (in engine control electronics rather than mechanical stuff, admittedly) for thirty years, this lack of reading may say something in itself. Oh, and the hottest thing in the combustion chamber is the spark plug tip. It's right where the action happens and has next to zero cooling. Unless you have a heavily coked up engine, in which case the stray deposits are made of a thermally non-conductive material and therefore get hotter than any of the metal anywhere. Link to comment Share on other sites More sharing options...
Clive Posted July 12, 2019 Report Share Posted July 12, 2019 For cooling, surely the inlets, and to a fair extent the sparkplug, are cooled by the cool fuel/air mix? The exhaust won't really benefit at all. And yes, multiple valves are the way forward. I am intrigued how modern engines happily use well over 10:1 cr with no issues, on 95 octane fuel. Must be combustion chamber shape, very controlled fueling and spot on timing. Link to comment Share on other sites More sharing options...
Pete Lewis Posted July 12, 2019 Report Share Posted July 12, 2019 Some waffle on valve numbers at a Cosworth talk depends on the chamber but 5 valves was the optimum to fully get the most , on what order of how many inlet vs exhaust I dont remember And Peugeot made a 16 valve engine in the early 1900s single cyl 1.5 litre . Nothing is new , Pete Link to comment Share on other sites More sharing options...
NonMember Posted July 12, 2019 Report Share Posted July 12, 2019 A five valve is always three in, two out. It probably is optimal, as Cosworth generally know what they're talking about, although what it's optimal for may not be typical road use... I thought the early 1900s 16 valve was a Skoda - although if Peugeot tried to squeeze 16 into one cylinder they were really pushing the envelope! Link to comment Share on other sites More sharing options...
Colin Lindsay Posted July 12, 2019 Report Share Posted July 12, 2019 My 1987 Rover 213s used to have a label on the engine claiming: "12 valve"... Link to comment Share on other sites More sharing options...
NonMember Posted July 12, 2019 Report Share Posted July 12, 2019 Yes, that's correct. The Rover 213 engine was a development of the Triumph Acclaim unit. It had two inlet and one exhaust valve per cylinder. Link to comment Share on other sites More sharing options...
rlubikey Posted July 12, 2019 Report Share Posted July 12, 2019 9 hours ago, NonMember said: I think John skipped a bit on explaining the area. The reason four-valve heads work .... Ahh! We'd moved on to 4-valve heads, thanks guys. In any case, I knew that filling only has 1-bar (normally aspirated) but exhaust has lots more. Not forgetting of course, that you have to empty the cylinder to make room for the inlet charge. I presume this is why the inlet is only slightly bigger than the exhaust, but that experiment/calculation is one I'm very happy to leave to others! Cheers, Richard Link to comment Share on other sites More sharing options...
ShaunW Posted July 12, 2019 Report Share Posted July 12, 2019 On 11/07/2019 at 15:29, NonMember said: Yes, as johny says. The mass of gas* going out is the same as that coming in, but it's got a LOT more pressure behind it. The mass of the gas may be the same but the volume is greatly increased. That's how it works. Small amount goes in, bang, it expands and pushes the piston down. That's how come pressure is created. The constant mass is reconfigured into different molecules that occupy a greater physical volume. Link to comment Share on other sites More sharing options...
JohnD Posted July 12, 2019 Report Share Posted July 12, 2019 Hmmmmmmmmmm! Yes, bit qquick on the draw about volume, I'll think again. But moderns and high compression ratios: Many work on a stratified charge. Rich at the plug for easy ignition, VERY lean elsewhere, for economy and low CO2. That enables high CR without pre-ignition. I think! JOhn Link to comment Share on other sites More sharing options...
ShaunW Posted July 12, 2019 Report Share Posted July 12, 2019 I'll take your word for it John. I was at the very limit of my dim and distant o-level physics so I'll bow out now. Link to comment Share on other sites More sharing options...
SpitFire6 Posted July 12, 2019 Author Report Share Posted July 12, 2019 Hi, I guess unusual valve timings & lift could mitigate this difference in ratio. I have yet to see an Internal Combustion Engine that did not have larger area inlet to the outlet. Cheers, Iain. 1 Link to comment Share on other sites More sharing options...
SpitFire6 Posted July 12, 2019 Author Report Share Posted July 12, 2019 (edited) 3 hours ago, rlubikey said: Ahh! We'd moved on to 4-valve heads, thanks guys. In any case, I knew that filling only has 1-bar (normally aspirated) but exhaust has lots more. Not forgetting of course, that you have to empty the cylinder to make room for the inlet charge. I presume this is why the inlet is only slightly bigger than the exhaust, but that experiment/calculation is one I'm very happy to leave to others! Cheers, Richard Hi. I surmise that you only get maximum flow at peak torque RPM. A larger inlet will increase the RPM where this is can be achieved. Cheers, Iain. Edited July 12, 2019 by Spitfire6 1 Link to comment Share on other sites More sharing options...
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