Peter Cobbold Posted March 19, 2015 Report Share Posted March 19, 2015 (edited) Edit: cracked it. ================================== Need some help from scientists and engineers please. Most websites covering boosted engine give tables of Effective Compression ratios like this: http://www.theblowershop.com/upload/Downloads/Effective_Compression.pdf I have done the calculations and they follow the formulae for adiabatic compression. Fine. Probem is this plot that appears to have originated in Allard's book shows eff Comp Ratios that are considerably less, two to three ratios less for a given boost: http://www.autocomponenti.com/boosted_tech/boosted_tech.htm After days of puzzlement I realise the plots have been made thus: The boosted compression ratio appears to have been calculated by taking the pressure ratio (boost psi + atmos psi) / atmos psi taking its square root and multplying the geom CR by that. eg 6psi boost gives a press rat of 1.41 sq rt of 1.4 is 1.19 So a CRstatic of 9 becomes a boosted CR of 9 x 1.19 or 10.71 and this is what we see on the plot. What I cant fathom is the justification for doing that. Is there anything in heat engines that obeys a square law relationship with compression? Peter Edited March 20, 2015 by Peter Cobbold Quote Link to post Share on other sites
pfenlon Posted March 19, 2015 Report Share Posted March 19, 2015 Peter if your struggling with something as simple as that! words fail me, the meaning of life is simpler. throw the blower away and get some cubic inches in there, with apologies for being a secondary school person. Good luck Quote Link to post Share on other sites
Dave Ashworth Posted March 19, 2015 Report Share Posted March 19, 2015 I think it is the square root that's at fault Peter. Other than ginger and mandrake most roots are roundish. Apologies for this childish and immature response, but with a lack of scientists and mathematicians throwing their hat in the ring, I felt I had to add something to acknowledge such a fine question. Dave. Quote Link to post Share on other sites
Peter Cobbold Posted March 20, 2015 Author Report Share Posted March 20, 2015 Pete, Dave, Well I wouldnt want TR-ers to be left out of the the fun that the Cobra owners have had with this topic: http://www.modularfords.com/threads/51059-Increasing-CR-vs-boost-pressure If the Allard plots are wrong then the Moss 6psi S/c kit on a 9.5:1static compression TR head is going to exceed the head's 10.5:1 knock limit at 2psi..... I think that is unlikely but want to know why. Peter (rooting for the squares) Quote Link to post Share on other sites
Peter Cobbold Posted March 20, 2015 Author Report Share Posted March 20, 2015 Sorted: https://supertrarged.wordpress.com/2015/03/20/effective-compression-ratio-and-boost/ Quote Link to post Share on other sites
GUY BENNETT Posted March 23, 2015 Report Share Posted March 23, 2015 Murphy's Law of Thermodynamics. Things Get Worse Under Pressure! Quote Link to post Share on other sites
Peter Cobbold Posted March 23, 2015 Author Report Share Posted March 23, 2015 (edited) It doesnt help Murphy that books can get boosted compression wrong. One recent book on Turbochargers (Hartman) states: "Suppose normally charged cranking yields 185psi and the turbo compressor adds another 15psi of boost pressure. The total compression component of cylinder pressure would be 200psi..." That's utter b*ll*cks, he's added the boost on after compression not before !! The correct pressure ratio is (14.7 +15)/14.7 which is 2. So the cranking pressure rises to 2 x 185 = 370psi. Not 200. Even Allard got it wrong: When describing compressor adiabatic efficiency ( Chap 2) "....if air entered the compressor at 20C and...was delivered from the outlet port at 1.5atmospheres and at 20C the compressor would be 100% adiabatically efficient ". No - that is impossible in thermodynamics. The air will be heated. Who needs Murphy when the books can sometimes mess up. Peter Edited March 23, 2015 by Peter Cobbold Quote Link to post Share on other sites
Cew Posted March 24, 2015 Report Share Posted March 24, 2015 This might help https://youtu.be/VnbiVw_1FNs Quote Link to post Share on other sites
Peter Cobbold Posted March 24, 2015 Author Report Share Posted March 24, 2015 Thanks Clive, Nothing quite like it for cooling the blood. Peter Quote Link to post Share on other sites
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