If gas engines were 100% efficient

[nboy22]

How long/far would we be able to go on a single tank of gas? I was just thinking about this the other day and I was wondering what the outcome would be.

 

[Howard]

Depends on the driving style, mass of the car, drag coefficient, etc. In fact, it's the same as asking how far you can get with an engine of n% efficiency, except that n is now 100 (or 1).

 

[natto fire]

Originally posted by: Howard
Depends on the driving style, mass of the car, drag coefficient, etc. In fact, it's the same as asking how far you can get with an engine of n% efficiency, except that n is now 100 (or 1).

Pretty much.

OP: Keep in mind it is not just the frictional losses in an engine that sap power. Although lots of the energy from the gasoline is lost through the tailpipe, and the cooling system (ICE's create loads of waste heat) there are also parasitic losses in the driveline, rolling resistance in the tires, etc. Air resistance is a big one though, as far as traveling at higher velocities.

To be honest, I think that combustion efficiency in ICE's are fairly respectable, considering what they where not more than 15 years ago.

Getting the rest of the parasitic losses down is a lot trickier, especially as the average consumer demands more and more from their automobile. Things like airbags, CD changers, etc., do not come free, and the bloating of automobile curb weights is plain evidence of that. The Prius is one of the very few automobiles that tries to save energy at every corner. It has a special insulated coolant tank that keeps the coolant warmer longer, saving the need for wasteful warmups. It also tries to only use the combustion engine at very high loads, to save on pumping losses, by the electronic throttle and IMA.

So, to reiterate, your question is rather vague, but does invite a lot of discussion. Maybe you should lock it down to a specific vehicle?

To try to put this in perspective, gasoline has about 34.8 megajoules of energy per liter. About 6.96 of that is delivered to the flywheel, which is sapped even more from the driveline losses mentioned earlier. ~12.53 of that is lost as thermal energy through the cooling system, whether through the radiator or HVAC system. ~13.2 is lost through the tail pipe as thermal energy, or unburned hydrocarbons, although the unburned hydrocarbon but has been declining for quite a while.

 

[Nathelion]

Gas engines have an efficiency of, ballpark, 50%, maybe a bit higher IIRC. So in broad terms, you would be able to to get somewhere under twice as far using a 100% efficient engine.

 

[Mark R]

Gas engines have an efficiency of around 30-35% under optimal conditions (wide open throttle near their maximum torque point). The efficiency drops precipitously as load on the engine decreases, so under 'typical' driving conditions efficiency is around 20%.

Diesel engines are more efficient due to their higher compression (perhaps up to 40% for truck sized engines) and because they avoid throttling losses - massive heavy diesels (for ships) have achieved almost 50%.

There's not a lot of simple engineering that can be applied to improve the efficiency of the engine. Hybrid technology allows the use of an engine which operates close to its maximum efficiency point for as long as possible - whereas in a conventional car the engine is hugely oversized to facility overtaking/acceleration, with a big efficiency penalty for cruising.

It should also be recalled that these numbers are the overall thermal efficiency - in practice it isn't even possible to approach 100%, as a 'perfect' engine would only achieve 'Carnot' efficiency (this is the theoretical maximum amount of heat energy that could be converted into mechanical work) which is around 70-80% for a typical engine.

The big energy drain in a car is friction - number 1 is drag due to air resistance, with rolling resistance quite a bit further down. Aerodynamic drag increases rapidly as speed increases, but is also dependent on the frontal area of the car and the car's shape. There have been a number of 'concept' cars which have had fantastically low drag coefficients - and this alone would probably have led to these cars having double, maybe even triple, the highway fuel efficiency of more conventional cars. However, these cars look 'weird' - they are low and sleek with a sort of spaceship quality, and their shape may mean they are relatively impractical.

 

[nboy22]

Originally posted by: Captain Howdy

Originally posted by: Howard
Depends on the driving style, mass of the car, drag coefficient, etc. In fact, it's the same as asking how far you can get with an engine of n% efficiency, except that n is now 100 (or 1).

Pretty much.

OP: Keep in mind it is not just the frictional losses in an engine that sap power. Although lots of the energy from the gasoline is lost through the tailpipe, and the cooling system (ICE's create loads of waste heat) there are also parasitic losses in the driveline, rolling resistance in the tires, etc. Air resistance is a big one though, as far as traveling at higher velocities.

To be honest, I think that combustion efficiency in ICE's are fairly respectable, considering what they where not more than 15 years ago.

Getting the rest of the parasitic losses down is a lot trickier, especially as the average consumer demands more and more from their automobile. Things like airbags, CD changers, etc., do not come free, and the bloating of automobile curb weights is plain evidence of that. The Prius is one of the very few automobiles that tries to save energy at every corner. It has a special insulated coolant tank that keeps the coolant warmer longer, saving the need for wasteful warmups. It also tries to only use the combustion engine at very high loads, to save on pumping losses, by the electronic throttle and IMA.

So, to reiterate, your question is rather vague, but does invite a lot of discussion. Maybe you should lock it down to a specific vehicle?

To try to put this in perspective, gasoline has about 34.8 megajoules of energy per liter. About 6.96 of that is delivered to the flywheel, which is sapped even more from the driveline losses mentioned earlier. ~12.53 of that is lost as thermal energy through the cooling system, whether through the radiator or HVAC system. ~13.2 is lost through the tail pipe as thermal energy, or unburned hydrocarbons, although the unburned hydrocarbon but has been declining for quite a while.

Well.. let's say a car that's 3000 pounds and has 24 MPG with the 35% effieciency, I guess if it was 100% efficient it would be close to 70 MPG..

But in all basics, a close to 100% efficiency is not plausible inside our universe, correct? unless there is some sort of physics that we don't know about, won't there always be something to hinder the efficiency?

 

[DrPizza]

I'm assuming that by 100% efficient, you mean you can use 100% of the gasoline's energy to propel the vehicle. I don't think it's useful to hypothesize that there's zero rolling resistance, zero air resistance, etc. So, as now, it'll still depend on the vehicle. Larger/heavier vehicles will still get lower mileage than lighter, more aerodynamic vehicles.

 

[nboy22]

Originally posted by: DrPizza
I'm assuming that by 100% efficient, you mean you can use 100% of the gasoline's energy to propel the vehicle. I don't think it's useful to hypothesize that there's zero rolling resistance, zero air resistance, etc. So, as now, it'll still depend on the vehicle. Larger/heavier vehicles will still get lower mileage than lighter, more aerodynamic vehicles.

Of course, I was just wondering more of a, "What if?" type of scenario.. Not realistic at all but I just wanted to know what would happen in such a situation.

 

[f95toli]

Originally posted by: nboy22

But in all basics, a close to 100% efficiency is not plausible inside our universe, correct? unless there is some sort of physics that we don't know about, won't there always be something to hinder the efficiency?

Correct, there is something called the Carnot limit which basically tells you the maximum efficieny of an engine; and it can never be 100%.

AFAIK modern engines are about as efficient as they can get. As has already been pointed out the "problem" is not so much the engine but the CAR itself (drag, friction etc).
Hybrids are better because they allow the gas engine to work near its optimum point but simply replacing an ordinary gas engine with a hybrid will probablyl not save you anywhere near 50%.
I.e the only way to really save fuel is to use hybrid engines n smaller cars with less drag and friction.

 

[CycloWizard]

Originally posted by: nboy22
Of course, I was just wondering more of a, "What if?" type of scenario.. Not realistic at all but I just wanted to know what would happen in such a situation.

If you assume that your current engine is 30% efficient, your new engine is 100% efficient, and all else being equal, then your future gas mileage could be estimated as current mileage * 100/30. So, my car got about 27 mpg this weekend (best it's gotten in 10 years ). If I could magically improve the efficiency to 100%, then it would increase to about 90 mpg.

 

[dmcowen674]

Originally posted by: f95toli

the only way to really save fuel is to use hybrid engines n smaller cars with less drag and friction.

That's why it irks me when the Oil Company supporters push for more and bigger SUV Hybrids.

 

[Idontcare]

Originally posted by: dmcowen674

Originally posted by: f95toli

the only way to really save fuel is to use hybrid engines n smaller cars with less drag and friction.

That's why it irks me when the Oil Company supporters push for more and bigger SUV Hybrids.

Yep, and then those same mysterious Oil Company supporters go out into suburbia and at gunpoint force those soccer moms to buy them.

The bastards, forcing consumers to buy things they don't want.

Where will it end - with governments requiring its citizens to give them a non-zero percentage of their income? The tyrrany of it all. We are all victims, everyone except the Oil Company supporters that is, they are never vicitimized by anyone, ever.

 

[Nathelion]

Could we please keep the sable rattling out of this forum?

 

[silverpig]

Originally posted by: Nathelion
Could we please keep the sable rattling out of this forum?

Who's flinging furs?


1. You can't win, you can only break even.
2. You can only break even at absolute zero.
3. You can't reach absolute zero.

 

[Howard]

Originally posted by: dmcowen674

Originally posted by: f95toli

the only way to really save fuel is to use hybrid engines n smaller cars with less drag and friction.

That's why it irks me when the Oil Company supporters push for more and bigger SUV Hybrids.

Do you know what forum you stumbled into?

 

[natto fire]

Originally posted by: nboy22
Well.. let's say a car that's 3000 pounds and has 24 MPG with the 35% effieciency, I guess if it was 100% efficient it would be close to 70 MPG..

But in all basics, a close to 100% efficiency is not plausible inside our universe, correct? unless there is some sort of physics that we don't know about, won't there always be something to hinder the efficiency?

Ok, to try to get back on topic, (thanks for the hijack Dave...) ICEs are air pumps, plain and simple. You can increase the volumetric efficiency with forced induction, such as with a turbocharger or supercharger. Volumetric efficiency basically refers to how well an engine can move the charge (which is the air/fuel mixture, consisting mostly of air) into and out of the cylinders. A turbocharger uses some of the waste energy from the exhaust to increase this number. A supercharger uses a pulley connected to the engine, creating another frictional parasitic loss, to increase volumetric efficiency. It should be noted that this number can and will be above 100% as that merely refers to atmospheric conditions. Since forced induction will create higher than atmospheric conditions in the intake manifold, you end up with a number greater than 100%.

This is the part where you remember percentages can mean whatever their creators want them to.

As for your second part, it would not be simple maths such as that, but yeah, it would be impossible to turn all of gasoline's chemical energy into torque used to turn the tires. Gas turbine engines are greater in this respect, but lack many features which make them unsuitable for automobile use.

In response to the diesel comment, diesel's have higher compression ratios because they rely on this compression to ignite the charge. Also, diesel has more chemical energy per gallon than gasoline, which helps explain it's higher MPG ratings. The longer stroke design of diesels give it lots of low-end torque which also helps with this.

Also, LOL at the 50% efficiency for reciprocating piston ICE, it is nowhere near that high.

 

[themisfit610]

I read a fascinating article about making use of the excess heat generated by internal combustion engines, particularly of the gasoline type.

Essentially, you have a six stroke engine, in wihch the first four strokes are the typical intake, compression, combustion, exhaust stages, but after this you have two stages where water is sprayed inside the piston to collect as much of the heat as possible, which therefore turns into steam, and forces the piston down again. This system has shown up to a 50% improvement in efficiency over an otherwise identical gasoline engine, though it shows only moderate gains in diesel engines.

Imagine a high temperature turbocharged engine that could somehow extract heat energy from the turbo and engine block! Maybe even cool the intake charge at the same time somehow?

~Misfit

 

[bobsmith1492]

Originally posted by: themisfit610
I read a fascinating article about making use of the excess heat generated by internal combustion engines, particularly of the gasoline type.

Essentially, you have a six stroke engine, in wihch the first four strokes are the typical intake, compression, combustion, exhaust stages, but after this you have two stages where water is sprayed inside the piston to collect as much of the heat as possible, which therefore turns into steam, and forces the piston down again. This system has shown up to a 50% improvement in efficiency over an otherwise identical gasoline engine, though it shows only moderate gains in diesel engines.

Imagine a high temperature turbocharged engine that could somehow extract heat energy from the turbo and engine block! Maybe even cool the intake charge at the same time somehow?

~Misfit

... Popular Mechanics?

Why even consider the 100% efficient engine when it is, as Mark pointed out, physically impossible? It would be more useful to consider an engine that is as efficient as physically possible.

 

[themisfit610]

That's where I read it! Thanks for the reminder

~MiSfit

 

[natto fire]

Originally posted by: bobsmith1492

Originally posted by: themisfit610
I read a fascinating article about making use of the excess heat generated by internal combustion engines, particularly of the gasoline type.

Essentially, you have a six stroke engine, in wihch the first four strokes are the typical intake, compression, combustion, exhaust stages, but after this you have two stages where water is sprayed inside the piston to collect as much of the heat as possible, which therefore turns into steam, and forces the piston down again. This system has shown up to a 50% improvement in efficiency over an otherwise identical gasoline engine, though it shows only moderate gains in diesel engines.

Imagine a high temperature turbocharged engine that could somehow extract heat energy from the turbo and engine block! Maybe even cool the intake charge at the same time somehow?

~Misfit

... Popular Mechanics?

Why even consider the 100% efficient engine when it is, as Mark pointed out, physically impossible? It would be more useful to consider an engine that is as efficient as physically possible.

Like an electric motor, and maybe using the braking energy and extra engine torque to recharge the batteries?

Really even a gas turbine engine is quite a bit more efficient than typical reciprocating piston engines. Rotary engines get a bit closer, but still can't get close to the max 90% efficiency of a gas turbine used with cogeneration.

Obviously because of the complexities and the fact that gas turbines really don't like to constantly adjust output power, they will not see use in a normal passenger car.

Even if they could make them smaller there would be the problem with the inability to change output power, so you would need one heck of a hydrostatic system for that, even thinking about using a normal disc clutch with that would be hilarious.

 

[Nathelion]

Possible one could take a page from the hybrid book and use the gas turbine to charge a battery that drives an electric motor. That might negate quite a bit of the efficiency advantage, though.

 

[Howard]

Originally posted by: Captain Howdy

Originally posted by: bobsmith1492

Originally posted by: themisfit610
I read a fascinating article about making use of the excess heat generated by internal combustion engines, particularly of the gasoline type.

Essentially, you have a six stroke engine, in wihch the first four strokes are the typical intake, compression, combustion, exhaust stages, but after this you have two stages where water is sprayed inside the piston to collect as much of the heat as possible, which therefore turns into steam, and forces the piston down again. This system has shown up to a 50% improvement in efficiency over an otherwise identical gasoline engine, though it shows only moderate gains in diesel engines.

Imagine a high temperature turbocharged engine that could somehow extract heat energy from the turbo and engine block! Maybe even cool the intake charge at the same time somehow?

~Misfit

... Popular Mechanics?

Why even consider the 100% efficient engine when it is, as Mark pointed out, physically impossible? It would be more useful to consider an engine that is as efficient as physically possible.

Like an electric motor, and maybe using the braking energy and extra engine torque to recharge the batteries?

Really even a gas turbine engine is quite a bit more efficient than typical reciprocating piston engines. Rotary engines get a bit closer

If you mean anything like a Wankel-cycle motor, that's 100% false.

 

[Mark R]

Originally posted by: Captain Howdy
Really even a gas turbine engine is quite a bit more efficient than typical reciprocating piston engines. Rotary engines get a bit closer, but still can't get close to the max 90% efficiency of a gas turbine used with cogeneration.

No way. Gas turbines are terribly inefficient, unless operated in some kind of combined cycle system where the waste heat is used for steam/vapour production.

While combined cycle turbines are used for power generation, simple gas turbines are only used for emergency or extreme peak power, or for cogeneration. Gas turbines only advantage are low cost.

Natural gas tankers were originally built with gas turbine engines, which could run off the vaporizing gas - but this has been abandoned because the terrible efficiency of the turbines made the ships too expensive to operate. More modern, or refitted, tankers use reciprocating piston diesel engines, which have nearly double the thermal efficiency.

Same is true for small scale power production from landfill or biogas - reciprocating piston engines are preferred, unless for cogeneration with an excess heat requirement.

 

[firewolfsm]

If we could develop a way to convert large amounts of heat into energy efficiency could go up 20%. It's being researched for electronics as a way of cutting down heat and improving battery life but it could be used for so many things. Currently they have about a 50% efficiency, imagine half the heat from cars going back as energy.

 

[highwire]

Originally posted by: firewolfsm
If we could develop a way to convert large amounts of heat into energy efficiency could go up 20%. It's being researched for electronics as a way of cutting down heat and improving battery life but it could be used for so many things. Currently they have about a 50% efficiency, imagine half the heat from cars going back as energy.

Mark R's good post near the top of the thread mentioned Carnot (Kar-no). But typical threads on these topics go on as though Carnot was a raving wino who left us nothing important to consider.

That is wrong. What is most directly to to point of this is his law that states the maximum fraction of work that can be derived from heat energy depends on the temp at which heat is introduced and how much that heat can fall while producing work before being rejected.

it is: (Tsource-Tsink) /Tsource

Turbos on diesels, high pressure ratios on fanjet engines help that fraction, but there is no way around it.

An example of Carnot applied: use the heat from the engines cooling jacket to get more power for free: (380°K-320°K)/380°K = 60/380= less than 16% of cooling jacket heat can be converted to power, as a realistic target, maybe 8%. So, with the additional apparatus required for this, half the size of the car itself, a couple more horsepower could be produced. Have I got some investors out there?

The point is this, low temp heat is junk heat .