Hello!
how can i count extra flow for:
E70 (70% of ethanol, 30% of gas)
E100 (100% ethanol)
Thank you!
OK. I'll try to put this in more common term for the ones that are not mathematical engineers like myself so that everyone can understand them and make use of it when applied to their car. I will use more common terms that will not seem cryptic.
Since the density of ethanol and gasoline is not the same (750kg/m3 for gasoline and 790kg/m3 for ethanol), the flow numbers will be off by a few percent when calculated with this easy formula that exludes the density. But it works fine for the sake of simplicity.
The volume of the resulting mix of fuels will not be the same either. If blending 1000cc of ethanol with 1000cc of gasoline you do not get 2000cc of fuel.
Then the viscosity is not the same either so an injector that has a specified flow for gasoline will not have the same flow for ethanol.
In the end the difference from the simple calculation compared to reality is around 2%.
To calculate the extra flow for any blend of ethanol and petrol:
Petrol, Lambda 1 = 14.7 AFR
Ethanol (E85), Lambda 1 = 9.765 AFR
Ethanol (E100), Lambda 1 = 9.0078 AFR
F1 = Gasoline AFR
F2 = Ethanol AFR
C1 = Gasoline content in %
C2 = Ethanol content in %
(F1*10000/((F2*C2)+(F1*C1)))-100
= The percentual increase in injector size
But usually you want to use injectors that have a little bit higher flow. This is because you should be able to use the cold start and acceleration enrichment in relation to the equivalence ratio in my table.
Let's take a look at it again.
Fuel ........................ AFRst ........ FARst ....... Equivalence Ratio ... Lambda
Gas stoich ................ 14.7 .......... 0.068 ................ 1 ................... 1
Gas max power rich .... 12.5 .......... 0.08 ................. 1.176 .............. 0.8503
Gas max power lean .... 13.23 ........ 0.0755 .............. 1.111 ............. 0.900
E85 stoich .................. 9.765 ....... 0.10235 ............ 1 ................... 1
E85 max power rich ...... 6.975 ....... 0.1434 .............. 1.40 ............... 0.7143
E85 max power lean ..... 8.4687 ...... 0.118 ............... 1.153 .............. 0.8673
E100 stoich ................ 9.0078 ...... 0.111 ............... 1 .................... 1
E100 max power rich .... 6.429 ........ 0.155 .............. 1.40 ................ 0.714
E100 max power lean .... 7.8 .... ...... 0.128 .............. 1.15 ................ 0.870
What this means is this:
Look at the blue numbers on both Gasoline and ethanol. Ethanol requires more fuel flow to get into the rich region, and therefore requires more fuel (percentual increase) at cold starts and when stabbing the throttle. Not much more than gasoline, but enough to maybe give driveability problems on some engines since they want to buck a little initially when the throttle is stabbed.
Let's look at the red numbers. In practice this means that to go full rich on gasoline you will need an extra 17.6% of fuel over stoich AFR for gasoline. But on ethanol you will need an extra 40% of fuel over stoich AFR for ethanol.
When using 42% bigger injectors which is the recommendation when going from gasoline to E85, the theory doesn't really add up and you should end up with a little bit too little enrichment when accelerating since the ECU will add 11.1-17.6% extra fuel. That is not enough fuel to enrich the mixture of ethanol which requires 15-40% extra fuel for enrichment.
In reality though, you will be pretty spot on the good numbers since our ECU's always want to enrich a little bit more than neccesary.
But it is still a good idea to use 10% larger injectors than calculated to make up for those situations when the ECU can not enrich properly.