I once read that a modern car engine is only about 30% efficient (that is, only about 30% of the nergy in the gasoline actually drives the wheels).
What interests me is the losses-how much is lost due to heat generation, friction in the drive train, etc.
How much loss is due to the reciprocating motion of the pistons and connecting rods? They must reverse direction after ech power stroke-is this an appreciable loss?
It puts electric cars in perspective-and all-electric car ought to be close to 90% efficient.
According to this graphic, internal combusion engines are 12.6% efficient if you define efficiency as the final energy available to move the wheels.
As to electrics, honestly I don’t know. There has been so much misinformation on both sides. It’s my understanding that electrics are much more fuel-efficient and nonpolluting than internal combustion engines, but when you take into account production and maintenance costs, they are more even (those big fancy exotic-metal batteries and control systems with their electronic computers and whatnot). I don’t presume to be the final authority on that, though. Cecil wrote a treatment of the subject a couple of years ago.
Going from memory, approx 30% of the energy in gasoline is converted to mechanical energy, the rest to waste heat.
Out of that 30% you have further mechanical losses to friction. There is no large loss directly from the fact that pistons reciprocate, losses are due to friction from piston rings (and some small losses in the bearings). About 80% of the mechanical power from the engine reaches the wheels.
The electric cars aren’t that impressive when you look at the entire power chain. Electricity has to be generated somewhere and somehow. That process is not 100% efficient. Then the electricity has to be transported to the car with more losses. Finally it has to be used to charge a battery which also has losses. You add all these losses up and depending on the original source for the electricity you could well end up with lower overall efficiency than a gasoline car.
True but if you are going to analyze upstream along the supply line, you also have to look at the inefficiencies of refining to produce gasoline, and transporting the gasoline to retail stations.
Yes. Comparing efficiencies depends on how the electricity was produced. If it came from coal, for instance, you should include transportation of the coal to the power plant. If it’s solar cells the amount of energy to manufacture them is extremely high. It gets confusing real fast, and getting a true (complete) efficiency figure is difficult. Yet, it’s dangerous to ignore the whole chain since the electric car has a great final efficiency, but is not as good ones you consider the whole power chain.
If you think about it, the energy efficiency starting from primary sources where humans first touch it to where it is ultimately used is probably really dismal.
internal combustion engines have a number of losses, all of which can be combined mathematically into one big “total energy in versus mechanical energy out” efficiency number, typically called “brake thermal efficiency.”
Combustion efficiency: modern IC engines are very good at releasing most of the thermal energy from the fuel. For gasoline engines a small amount of fuel mixture escapes combustion by hiding right next to the combustion chamber wall and down near the piston rings, but generally 95-98% of the available thermal energy is released.
Indicated efficiency: This number describes how much of the fuel’s energy gets converted to mechanical work by the piston. There are several factors that affect this number:
[ul]
[li]combustion efficiency (see above). If the fuel hasn’t been burned, there’s no way to extract mechanical work from it.[/li]
[li]combustion timing. There’s a best spark timing (or for diesel, injection timing) that allows the piston to extract the most possible mechanical work from the resulting hot mixture. Because of emissions regulations, diesels generally can’t inject at this optimal timing because they will produce to much NOx. Gasoline engines may not be able to spark at this timing because of problems with knock. [/li]
[li]compression ratio. The higher the compression ratio, the greater the percentage of the fuel’s thermal energy can be extracted as mechanical work. Once you’ve set the compression ratio during the design of the engine, the laws of thermodynamics dictate the absolute maximum efficiency of the engine. For a diesel engine operating at ~19:1 compression, this limits the maximum efficiency to no more than ~60%.[/li]
[li]Heat transfer. Your car has a big-ass radiator up front. All of the heat that it dumps to atmosphere originally came from gasoline, heat that was lost to the walls of the combustion chamber after each combustion event, during the expansion stroke. With higher combustion temperatures that occur at higher load settings, more heat is lost to the combustion chamber walls, but of course this is offset to a large degree by the engine making more power. Diesel engines tend to lose a smaller percentage of heat to the combustion chamber walls than gasoline engines because they run lean: there’s a lot of excess air in the combustion chamber, and that keeps peak combustion temps lower. In addition to higher compression ratios, this is another reason diesel engines are typically more efficient than gasoline engines. A larger combustion chamber has a lower surface-area-to-volume ratio and so typically loses a smaller percentage of energy to the combustion chamber walls. This is one reason why gigantic marine diesel engines, with pistons about 3 feet across, are more efficient than passenger-car diesels.[/li][/ul]
Hot exhaust. Because the expansion stroke does not extract all of the thermal energy from the hot combustion products, the remainder of the energy leaves the engine as hot exhaust.
Mechanical losses. Of the mechanical energy imparted to the piston from the combustion products, a portion goes to keeping the engine running. Some is lost to friction in the bearings, and some drives things like the water pump, alternator, cooling fan, oil pump, fuel pump, etc. These can vary significantly depending on RPM, but under highway cruise conditions may consume 10% of the mechanical power provided to the crankshaft by the piston.
Pumping losses. In a gasoline engine operating at wide-open throttle, the losses associated with engine breathing aren’t terribly large. However, at part-load, they can be significant. It’s worth noting that your car’s engine is operating at part load virtually all the time. Pumping losses are generally pretty small in a diesel engine, which has no throttle plate to restrict incoming air; it’s a third reason why diesels are typically more efficient.
So the crude rule of thumb is that of the fuel energy that gets put in, 1/3 gets dumped as heat from the radiator, 1/3 gets dumped as hot exhaust, and about 1/3 makes it to the crankshaft as mechanical work. But this varies hugely depending on engine design and operating parameters. A Volkswagen TDI from about ten years ago was able to hit about 42% overall mechanical efficiency. However, this was only true for a narrow range of operating conditions, and probably is not true for more recent versions, which are subject to far more stringent NOx regulations that require less-than-optimal fuel injection timing (Volkswagon Jetta from back then got about 60 MPG highway, nowadays it’s about 40).
Because of lean combustion and the absence of a throttle plate, diesel engine efficiency doesn’t drop off very rapidly (from maximum) at part-load operation; passenger car diesel engines may conceivably be operating at 25-30% efficiency during steady-state highway cruising. The situation is worse for gasoline engines: depending on their size, they may be operating at a very small percentage of full load when at highway cruise, and efficiency could be 15-25% under these conditions. Efficiency is even lower at the power levels required for city cruise.
The reciprocation of the pistons and rods doesn’t waste energy, but it does transfer kinetic energy back and forth between it (the piston) and the crankshaft. At mid-stroke when the piston is moving very fast, it’s got a lot of kinetic energy; it got that energy from the crankshaft and flywheel. at TDC and BDC, when the piston is barely moving, that’s because it has been decelerated by the crankshaft; all of that kinetic energy now resides in the slightly faster rotation of the crankshaft/flywheel assembly. So there’s an inherent unevenness of instantaneous engine RPM due to piston reciprocation, although this is hardly noticed next to the unevenness due to compression and expansion events. All of this is smoothed out by having a multi-cylinder engine, and/or having a massive flywheel on the crankshaft.
The 90% efficiency figure you quote for electric cars doesn’t tell the whole story. An electric power plant that burns fossil fuel or nuclear fuel typically spins a turbine/alternator with steam heated by the fuel. Similar to your car’s engine, the steam operates in a cycle with thermodynamic parameters that dictate its maximum efficiency, typically around 50%. Then there’s transmission losses involved in getting that energy down the wires to your home. My understanding is that the overall efficiency of burning fuel at the power plant and getting energy from your electrical outlet is about 30%. Your electric car may be about 90% efficient in getting that energy to its wheels, so overall system efficiency would be about 27%, on par with gasoline or diesel engined cars. But the electric car displaces its pollution to a remote site (the power plant), and the stationary power plant can also implement more exacting pollution controls. Moreover, the electric vehicle reduces demand for petroleum fuel. And the electrical power may also come from renewable sources, such as solar, wind, and hydro.
Hi, I’m trying to do mathematical modelling of rotary and reciprocating engines. Machine Elf mentions that the reciprocating piston and rods don’t waste energy, that it is merely transferred back and forth through the crankshaft. But this is the longitudinal motion occuring - the up and down motion of the piston, conrod and crankshaft. What about the lateral (side to side) motion of the conrod, crankarm anf joining pin? I can’t find anything on the net that talks about this or accounts for it in the losses. I have done some calculations and think that it accounts for about a quarter of the gross work done by the piston. But this seems to be an unusually large loss not to be mentioned anywhere! Can anyone give me some clues?
Rotary Engine Man
Wow, a little blast from the past. It’s funny looking back at my posts above considering that I now drive a Tesla.
Since 2011 I’ve read some studies claiming that the “well-to-wheel” energy usage (measured as CO2 emissions) for an EV is significantly better than ICE, by about a factor of 2:1 for the worst case (coal powered electricity plant). Sorry, no cite. They claimed to have included battery manufacturing for the EV, as well as pumping oil from the ground, transporting, refining etc for the ICE.
One little tidbit that may or may not help you: Piston engines also have pumping losses. The air that is under the piston as it moves down has to be moved to another cylinder where the piston is going up. Race engines often have vacuum pumps that decrease air pressure in the crank case to reduce these losses and they see measurable power gains on the dyno.
No loss. The big end of the rod isn’t moving from side to side, it’s moving in a circle along with the crankpin on the crankshaft. The small end of the rod is reciprocating along with the piston. And no matter how it moves, if its movement is externally imposed by mechanical connections at the crank and piston (instead of by some dissipative/braking mechanism), then there’s no loss involved; a change in velocity simply means energy has been transferred to somewhere else in the mechanism.
If an engine were somehow made to be absolutely frictionless, you could give the flywheel a spin and it would keep spinning forever, with pistons reciprocating and rods doing their booty-dance. The losses are in the bearings, piston/bore sliding, and air movement.
“Pumping losses” is a term that has very specific meaning where IC engines are concerned: it refers to the losses incurred when moving combustion air past a restrictive throttle plate and/or restrictive intake/exhaust valves. Air getting churned around in the crankcase may cause measurable/meaningful losses for race engines spinning at stratospheric RPM, but for a family sedan cruising down the highway at 2500 RPM, that loss is negligible.
Except for the losses , friction between surfaces, and the drag caused by the parts having to move through air, and the bearings having to move through grease or oil.
Agreed - but my point is that the reciprocating motion of a piston is not inherently any more lossy than the rotating motion of a crankshaft; the kinetic energy of a piston at mid-stroke (when it’s at its highest speed) is not pissed away as heat when it gets to TDC/BDC (when it reverses direction), it’s simply stored somewhere else for a short time.