Alcohol Distillation

Ethanol fuel is made by fermenting a sugar or starch mash to a wash of 8 to 15% alcohol and distilling it until it is strong enough to burn cleanly in an engine, which the corpus puts at 75% or better and, in its dedicated fuel article, at 85 to 95%. What you make is fuel and not drink: methanol is spread through almost every fraction of a run, so a foreshots cut does not remove it, and no volumetric rule makes a home distillate safe to drink.

Part of Internal Combustion

Making ethanol engine fuel by fermentation and distillation, how strong it must be, what cuts mean for fuel, and the fire, gas and poisoning hazards.

Fermentation Makes A Weak Wash And Distillation Concentrates It

Yeast turns sugar into ethanol and carbon dioxide, at most 0.51 kg of ethanol per kilogram of sugar, as the corpus gives in Fermentation. The carrier’s wash is 8 to 15% alcohol, depending on the sugar and the yeast. Starch must first be converted to sugar by mashing, as Grain Mash and the carrier describe.

Distillation uses the difference in boiling points: ethanol boils at 78.4 °C and water at 100 °C, so heating the wash drives off a vapour richer in alcohol, which a condenser turns back into liquid. A single pass through a pot still gives 40 to 60%, and a second run reaches 70 to 80%, according to Column Still vs Pot Still; a packed reflux column reaches 92 to 95% in one pass. Building a still is covered in Still Construction, and the overall chemistry in Alcohol and Distillation.

How Strong The Fuel Must Be

The corpus gives two answers. The carrier says engine fuel needs at least 70% (140 proof), with a target of 75% or better and 90% running cleanest. Ethanol Fuel Production says engine fuel needs 85 to 95%. Dehydration to Fuel Grade explains that the 95.6% azeotrope is the limit of ordinary distillation, and that hydrous 95% ethanol will not blend with gasoline. Where two corpus figures disagree, use the higher unless the engine has been shown to run well on less: water in the fuel lowers the heat released, makes starting harder and corrodes parts.

For Fuel, Cuts Are Beside The Point; For Drink, No Cut Makes It Safe

An earlier version of the corpus told readers to discard the first 50 to 100 ml of a run because methanol in the foreshots would blind them. That is withdrawn. Methanol Hazards is now the authoritative article, and its position is:

• Methanol is not concentrated in the early fraction. Blumenthal and colleagues (Molecules 26(9):2585, 2021) found it in “almost equal concentration in almost all fractions”, accumulating in the tailings, and traced the idea that it is enriched in the heads to a translation mistake.
• The foreshots cut is a flavour practice. The sharp smell of the first jars is acetaldehyde and ethyl acetate.
• The feedstock is the control. The EU permits 30 g of methanol per hectolitre of pure alcohol in neutral spirit against 1,000 to 1,500 for fruit and pomace spirits.

For an engine, none of this matters, because methanol burns. You can discard the first 50 to 100 ml per 20 litres of wash to keep the batch cleaner, or keep it; either way it burns. The real warning is the opposite of the old one. Fuel ethanol is not drink. It can carry methanol, fusel oils and whatever was in the vessel, it may have been denatured on purpose, and no volumetric rule makes a home distillate safe to drink. Potable spirit is a different subject with different requirements.

What You Need

• A fermentation vessel with an airlock, sugar or starch, and yeast.
• A still and condenser, built for the job and tested with water first.
• A hydrometer to read strength, which is meaningful only for alcohol and water.
• A heat source that does not put a flame under a vapour leak. A water bath or steam jacket is safer than direct fire.
• Containers that suit ethanol and a label: “FUEL, NOT FOR DRINKING”.

Method: From Mash To Fuel

Step 1: Ferment. Keep the mash at 20 to 30 °C, the carrier’s range, and let it finish. When bubbling at the airlock stops, fermentation is done.

Step 2: Distil slowly, outdoors. Fill the pot about two-thirds full, as the carrier says, heat gradually, and keep cooling water flowing through the condenser.

Step 3: Keep the strong fraction and rerun the weak. Watch the vapour temperature. The carrier collects at 78 to 82 °C, notes that the water content rises above 85 °C, and stops collecting when the temperature settles above 90 °C; a second run raises the strength. Corpus articles differ on the temperatures to expect, because they depend on where the thermometer sits, so judge by the strength you measure.

Step 4: Read the strength and dry if necessary. Compare with the target above, and see Dehydration to Fuel Grade for going past the azeotrope.

Step 5: Label and store as fuel. Away from flames, away from children and from drinks, in a labelled container.

Worked Example: What A Batch Is Worth

Take 20 litres of wash at 10% alcohol, an assumed figure inside the carrier’s 8 to 15% range.

• Pure alcohol in the wash: 20 × 0.10 = 2 litres.
• At best, if all of it is recovered at 85% strength: 2 ÷ 0.85 = 2.35 litres of fuel.
• Replacing petrol at 1.5 litres of ethanol per litre (the carrier’s ratio, taken here as pure ethanol): 2 ÷ 1.5 = 1.33 litres of petrol replaced.

So a whole 20-litre batch, with perfect recovery, replaces about a litre and a third of petrol. Real recovery is lower. The arithmetic shows why ethanol fuel needs real crop production, not leftovers, as the carrier says.

Common Mistakes

• Treating the foreshots cut as a safety step. It is for flavour only.
• Drinking fuel ethanol, or giving it to anyone. It is not made or tested as a drink.
• Putting a naked flame under a leaky still. Ethanol vapour ignites between 3.3 and 19% in air.
• Letting the condenser water stop. The vapour escapes unburned into the room.
• Fermenting in a closed room. The carbon dioxide pools.

Safety

Fuel ethanol is not drink, and the vapour burns invisibly

No volume rule makes a home distillate safe to drink. Label every container as fuel. Ethanol vapour ignites at 3.3 to 19% in air from a flash point of 13 °C and burns almost invisibly, and 2 litres in a small closed room reaches the limit, according to Fire Safety: distil outdoors, use no direct flame, and keep a bucket of sand and water to hand. Fermentation gives off carbon dioxide, about 4.2 m³ from a 100 litre wash according to Fermentation, and it must not pool in a low or closed space. Methanol is toxic by skin and breath; see Methanol Hazards.

What’s Next

• Learn next: Carburetor Design — metering the fuel into the engine
• Combine with: Ethanol in Gasoline Engines — the effect of ethanol on an existing engine