A gasoline engine takes 10-20% ethanol in its fuel with no changes in the parent’s view, and the ethanol raises octane and stretches the gasoline, but it leans a fixed-jet carburettor by about 7% at E10 and 13% at E20, so a carburetted engine may need richer jets. The ethanol must be dry or it separates from the gasoline, and the fuel system must be ethanol-resistant. Cold starts below about 10°C are poor and are best made on gasoline.
Part of Ethanol Fuel Production
Running a spark-ignition engine on gasoline-ethanol blends or on neat ethanol, and the effects on mixture, power, starting and materials.
Blends Stretch Gasoline Without Changing The Engine Much
The parent gives ethanol an octane rating of 108-110 RON, so it resists knock even at high compression. A blend of 10-20% ethanol in gasoline (E10-E20) is described as needing no engine modification, which makes it a way to stretch a limited gasoline supply. E85 (85% ethanol) is the flex-fuel blend and needs the neat-ethanol changes (Carburetor & Injection Modifications).
The Mixture Goes Lean As Ethanol Goes Up
Ethanol carries oxygen in its own molecule. Gasoline needs about 14.7 kg of air per kg for a chemically correct burn (stoichiometric), and ethanol about 9.0. Mixing by volume with densities of 0.74 kg/L for gasoline (assumed; 0.72-0.78 is the usual range) and 0.789 for ethanol:
| Blend | Ethanol by mass | Air needed (kg/kg fuel) | Fuel needed vs gasoline, same air | Energy per litre |
|---|---|---|---|---|
| E0 | 0% | 14.7 | 1.00 | 32 MJ |
| E10 | 10.6% | 13.8 | 1.07 | 30.9 MJ |
| E20 | 21.0% | 13.0 | 1.13 | 29.8 MJ |
| E85 | 85.8% | 9.5 | 1.54 | 22.7 MJ |
Working E10: ethanol mass 0.1 × 0.789 = 0.0789 and gasoline 0.9 × 0.74 = 0.666; ethanol share 0.0789 ÷ 0.745 = 10.6%. Air demand = 1 ÷ (0.894/14.7 + 0.106/9.0) = 1 ÷ (0.0608 + 0.0118) = 13.8. The ratio 14.7 ÷ 13.8 = 1.07, meaning 7% more fuel mass is needed to stay on the chemically correct mixture.
An engine with a mixture sensor and computer adjusts for this. A plain carburettor does not: with its gasoline jets it runs 7% lean on E10 and 13% lean on E20 (14.7 ÷ 13.0 = 1.13). Lean running overheats exhaust valves and causes hesitation and backfire. So the parent’s “no modifications” holds for fuel-injected engines with feedback, not safely for every old carburettor; test E10 first and watch for lean symptoms before going to E20.
What You Need
- Dry ethanol, since blends only stay mixed when the water content is low (Dehydration to Fuel Grade). Hydrous 90-95% ethanol and gasoline separate
- Fuel lines and seals that survive ethanol (Fuel Line Material Compatibility)
- A way to start on gasoline for cold mornings: the parent notes ethanol starts poorly below about 10°C because of its low vapour pressure, so keep some gasoline and switch over once the engine is warm
Method
Step 1: Check the fuel system for rubber (hoses, diaphragms, seals) and replace what ethanol attacks.
Step 2: Fill with a small trial blend (E10), mixing ethanol into gasoline in the tank or in a clean can, shaking, and waiting a short time to confirm one clear liquid.
Step 3: Run it under load and listen for pinging or hesitation. Look for white-grey spark plugs, a sign of lean running.
Step 4: Step up to E20 if the engine is happy.
Common Mistakes
- Using hydrous ethanol in a blend. Water drops out as a bottom layer that the engine sucks up and stalls on.
- Leaving ethanol blends in a tank for months. Ethanol absorbs water from air and the tank contents can separate.
- Assuming octane means more power. The engine only gains if it has been set up to use it (higher compression, advanced timing).
Ethanol and the engine bay
Ethanol’s flash point is 13°C, so vapour ignites at room temperature and a pale, near-invisible flame can burn without being seen in daylight. Keep leak checks regular, carry sand or a fire extinguisher, and see Fire Safety. Exhaust gas contains carbon monoxide.
What’s Next
- Learn next: Carburetor & Injection Modifications, to go beyond E20
- Combine with: Engine Use, for the whole picture