Small amounts of ethanol are considered safe to run in most vehicles. However, ethanol is more corrosive and burns hotter than gasoline which could cause some engines to misfire, stall or overheat. It also emits more nitrous oxide and formaldehyde than gasoline and could potentially damage fuel pumps, lines, fittings and tanks
Actually ethanol or alcohol burns much cooler than pure gas. Pure gas has a much higher btu content and molecule for molecule makes more power whereas heat = power. Stoichiometric value for alcohol is 9:1 vs 14.7:1 for pure gas thus it takes 1.6 time as much alcohol or ethanol(fancy word for grain alcohol) to make the same amount of heat. Ever wonder why you get better fuel mileage running on pure gas?
The general consensus is ethanol burns “cooler” than gas. Technically that is correct but what does that really mean? There’s a lot more to it and I’m throwing out my analysis (btw, this is not copied from any textbook or external resource--just my own back-of-the-envelope calcs so I make no assurances).
The burning question in my mind has been how does cooler burning correlate to cylinder temps and power? That is, how can a fuel that burns “cooler” generate more power? How much “cooler” does it burn? Are the EGTs cooler? To explain this, I did some calcs--the results seem reasonable so I thought I would post them here with explanation.
If you take one mole (6.02 x 10^23 molecules) of octane (gasoline) and one mole of ethanol and combust them in air, octane would produce 5460 KJ of heat compared to 1368 KJ for ethanol (eqs 1 and 2). The heat given off from the combustion reaction can be experimentally measured to a very precise number. In this case, more heat is evolved from combusting one mole of octane than one mole of ethanol, in fact, about 4 times as much.
C8H18 + 12.5O2 → 8CO2 + 9H2O DH (heat released) = -5460 KJ/mol [eq 1]
C2H5OH + 3O2 → 2CO2 + 3H2O DH = -1368 KJ/mol [eq 2]
(note: negative sign just means heat is released; more negative = more heat)
So in this sense, ethanol burns cooler.
Now, let’s take a look at what happens inside the cylinder when the oxygen content is the limiting factor. For this, we need to look at the combustion equations where the oxygen content is set as the limiting reagent (i.e. 12.5 moles). For every 12.5 moles of O2, you can burn 1 mole of octane and 4.16 moles of ethanol. So you can see in this case, you actually get more heat from ethanol per unit oxygen (approximately 4.4% more) [eq 3].
C8H18 + 12.5O2 → 8CO2 + 9H2O DH (heat released) = -5460 KJ/mol
4.16C2H5OH + 12.5O2 → 8.3 CO2 + 12.5 H2O DH = -5700 KJ/mol [eq 3]
If we scale down the last equation by 4.4% such that the thermal energy produced from burning ethanol equals that of octane, the equation becomes
3.98 C2H5OH + 11.97 O2 → 7.95CO2 + 11.97 H2O DH = -5700/1.044 = -5460 KJ/mole [eq 4]
So how does ethanol burn cooler, yet produce more power? Well, power is a result of cylinder pressure. For that, we need to take into consideration the total number of moles of combustion products from octane and ethanol. For octane, you get a total of 17 moles of combustion products (eq. 1). For ethanol, you get a total of approximately 20 moles of combustion products (eq 4). That corresponds to approximately 18% more moles of exhaust products from burning ethanol at the same thermal energy level as octane.
Since P=nRT/V, pressure is proportional to not only T (temperature) but also n (# of moles of total exhaust products) at constant V. Well, if EtOH produces 18% more moles of gas at the same thermal energy level as octane, then the temp can drop by 18% to produce the same cylinder pressure. Hence, ethanol can burn cooler to give the same pressure as burning octane because it produces a greater amount of combustion products.
Here’s the rub--everyone wants max power, so you end up burning as much ethanol as there is O2 in the cylinder which thereby produces 4.4% more heat than octane with even greater cylinder pressures. So while ethanol can burn cooler and produce more power, in reality, we end up burning as much ethanol as possible to get max power. This results in higher cylinder temps than gas (but more power too ).