E85 with Direct Injection explained!!!
Let’s talk about E85 first!
Ethanol based fuels are common place in the automotive world and since ethanol is a renewable resource and a cleaner burning alternative to gasoline, ethanol, or mores specifically ethanol blends, have become a popular choice in many countries. Pure ethanol or E100 isn’t typically used as a fuel since its lack of volatility (compared to gasoline) can make it hard to start when the engine is cold. The most common blend we are used to is referred to as E85, consisting of 85% ethanol and 15% unleaded gasoline. It’s worth noting however that the actual ethanol content of E85 may fluctuate quite widely in pump E85 – For example it’s common to see ethanol content in a winter blend of E85 reduce to perhaps 60-70% to aid starting in very cold climates.
While E85 is great for the environment, performance enthusiasts quickly found out it’s also great for producing power. So where does the advantage come from? On the face of it ethanol actually has a slightly lower energy content than gasoline per kilogram, but it also has a much richer stoichiometric AFR (9.8:1 for E85 vs 14.7:1 for pump gas) which means we need to use more of it to mix with the same mass of air. In general when switching to E85 we will find that we need to inject around 35-40% more fuel to make the same power we saw on pump gas. E85 also contains around 30% oxygen by weight and it has a higher effective octane rating than pump gas – This is often listed as 105, however we have seen E85 perform as well as specifically blended race fuels with motor octane ratings of 116+
One of the key advantages of E85 over gasoline is that it has a higher latent heat of evaporation than pump gas. This means that it absorbs more heat from the combustion charge as it goes through a phase change from liquid to vapor. This draws heat out of the combustion charge and coupled with its high octane rating, makes the fuel very resistant to detonation. For those interested in performance, this means we can run more boost, more compression, more ignition advance, or all three with relative immunity from detonation. Note that contrary to popular belief, knock can still occur on E85, but it is much less likely.
The lower energy density of E85 requires 30–40% increased flow from the fuel system to maintain proper lambda. Usually, fuel injectors and fuel pumps—and occasionally fuel lines—need to be upgraded to increase flow. Selecting an injector size that meets the flow requirements while maintaining a 90% or lower duty cycle is recommended. It is also recommended to have 10–20% headroom on the fuel pump.
As you can see; E85 has some great attributes for making power but the big issue lies in the amount of total fuel mass needed to make and that ethanol needs 30-40% more volume compared to gasoline. This puts a DI system, which is usually at its end in really bad shape!
In a DI system; the calibration has parameters for Start Of Injection. Once this is established the ECU will keep spray to hit a target request of fuel. If this request can’t be met, the system will continue trying to feed the engine fuel to the point the rail pressure starts to drop and extends the End Of Injection into the compression stroke of engine.
This is a bad place to be as this will trigger significant misfires in the combustion chamber and also lean the engine out for fuel supply.
So relative to the Direct Injected GM Engines
LT1 Engines Normally Aspirated Can run 100% E85
LT1 Engines with Forced Induction cannot run 100% E85
LT4 Engines with stoc boost Can run 100% E85
LT4 Engines with more the stock boost cannot run E85!!