Compression Ratio/fuel Octane

[Kane 34]

After a bit of clarification regarding fuel octane and dynamic compression ratio. Been trawling through the internet but unfortunately the figures I've come across seem to vary from anywhere between 8 and 9.5.

 

Based on UK super unleaded (98 RON) what would be a recommended target DCR to shoot for?

[petert 601]

Low 8's max.

[Kane 34]

Cheers Peter.

 

Would i be correct in assuming that there is some reservation in the max value of say 8.0-8.2 to take into account ambient conditions. For example if a theoretical engine was running 8.2 DCR is there the liklihood of it experiencing detonation if inlet temps were above average, say on a hot day?

 

This is probably not a concern in my situation being in Scotland but in theory would you look to reduce that figure further to say high 7's if that instance was likely to occur?

[welshpug 1,666]

how do you work out dynamic compression?

[Biggles 65]

It's CR based on when the inlet valve closes rather than BDC. Using an extreme example, if your static CR is 10:1 but the inlet valve doesn't close until 90 deg ABDC, dynamic CR is only 5:1.

[Kane 34]

Yeah so as biggles says instead of using the full volume of the cylinder like you would on a static CR you have to determine when after BDC the valve is actually closed and subsequently the new volume of the cylinder due to the piston being further up the bore.

[Biggles 65]

What CR you can run is influenced by the type of engine and any mods you might have made so there is no hard & fast number. The range of 8 to 9.5 sounds right to me. I've had engines that have been high 8's / low 9's (none 205 engines btw) and a friend's short stroke 2L Duratec is 9.5. I've seen some figures suggesting higher CR's but nothing I've ever verified.

Edited October 28, 2015 by Biggles

[Kane 34]

Although the engines you mention have run with higher figures doesn't necessarily mean that they are performing at their optimum efficiency on that fuel though wouldn't you say? Obviously you will know more than me about the specific engines but to enable them to run higher compression on similar fuel was there any requirement for the ignition to be adjusted to prevent detonation?

 

I would of thought that there would be a fairly accurate method of determining the absolute limit of dynamic compression for a given fuel since detonation is dictated by pressure and heat which obviously increases in line with a rise in compression.

[petert 601]

Cheers Peter.

 

Would i be correct in assuming that there is some reservation in the max value of say 8.0-8.2 to take into account ambient conditions. For example if a theoretical engine was running 8.2 DCR is there the liklihood of it experiencing detonation if inlet temps were above average, say on a hot day?

 

This is probably not a concern in my situation being in Scotland but in theory would you look to reduce that figure further to say high 7's if that instance was likely to occur?

No, the change in pressure from air temp is minimal compared to the inlet closing time. Variations in air temp should be catered for in the air temp vs ignition timing map. Don't let ignition timing influence the build. You give the engine what it needs, not what you think it needs, by monitoring the torque output on the dyno at individual load points.

[Kane 34]

I notice that cam manufacturer's typically only provide inlet closing angles ABDC at 1mm lift. Does anyone know the method for determining the actual angle ABDC that the valve is closed?

[Biggles 65]

Although the engines you mention have run with higher figures doesn't necessarily mean that they are performing at their optimum efficiency on that fuel though wouldn't you say? Obviously you will know more than me about the specific engines but to enable them to run higher compression on similar fuel was there any requirement for the ignition to be adjusted to prevent detonation?

 

I would of thought that there would be a fairly accurate method of determining the absolute limit of dynamic compression for a given fuel since detonation is dictated by pressure and heat which obviously increases in line with a rise in compression.

Depends what you mean by optimum efficiency. I never ran any of them with cylinder pressure sensors so don't know if the 50% burn point was at 8 deg ATDC across the load range. However, one of the engines in the low 9's was a production engine so I would expect the OEM would have done exactly that. The engine in question was optimised for fuel economy (ie part load) so I would not be surprised if it was configured with a high CR (to give max economy) but in the knowledge that at high speed / high load the ignition timing would be compromised.

 

Ref your absolute limit for CR - it's not quite that simple - on a given fuel, your absolute CR limit for an A Series engine will be noticeably lower than for a BMW M54 (for example). The combustion chamber shape has an influence in the form of localised hot spots and the A Series is a fine example of how not to do it. Conversely, BMW combustion chambers are an example of excellent (if expensive) practice. Then, of course, you can further complicate things by adding direct injection into the equation...

[Kane 34]

Thanks for informative response. This is a completely new subject area that I'm just beginning to look into so any advice/information is much appreciated.

 

I guess that the previous low 8's figure mentioned would be a fairly reasonable guideline if no other information was considered i.e. combustion chamber design.

 

Looks like more reading is in order to get my head round this..

[petert 601]

I notice that cam manufacturer's typically only provide inlet closing angles ABDC at 1mm lift. Does anyone know the method for determining the actual angle ABDC that the valve is closed?

 

Unless provided, it's normally derived from the "advertised" duration, once a lobe centre line is decided.

 

eg a 280 deg cam on 108 LCL

 

inlet opens 32 BTDC

inlet closes 68 ABDC