Atkinson Thermal Efficiency

Hey Guys,

Did some searching both on google and here and can't seem to find a nice comparison between an Atkinson cycle and a diesel cycle. So I'm interested in raw thermal efficiency numbers. Given the same displacement which would have a higher conversion of energy?

Gideon

Prius versus TDI, eh?

First and second generation Prius (Atkinson cycle) - 37% thermal efficiency.
Third generation Prius - 38% thermal efficiency.

Passenger car diesel engine (Diesel cycle) typical thermal efficiency ~ 34%.

Passenger car gasoline engine (Otto cycle) typical thermal efficiency ~ 28%.

If you want more than that you'll need to make a quick pitstop for a mechanical engineering degree.

ALH and Common Rail TDI BSFC chart about 41-42%Thermal efficiency for the 1996-2012 MY's give or take.



Prius BSFC chart



Older Mercedes 300SD



Lowest number wins... (fuel per kW per hour)

wow cool. so we still do better on thermal efficiency. Thanks so much

gidb36 said:

wow cool. so we still do better on thermal efficiency. Thanks so much

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Yes and no. In theory, yes. In practice, not by much. Drivbiwire's number of 41 to 42% for ALH and common rail engines is close to the maximum theoretical thermal efficiency for these engines and won't be achieved in practice by anyone except a dedicated hypermiler. 41 to 42% thermal efficiency for a diesel engine equals a continuous (average) BSFC of 208.

The Atkinson cycle engine in the Prius has the advantage of being operated to maximize its thermal efficiency. Looking at the BSFC maps posted by drivbiwire you can see the Prius engine is operated so its BSFC is about 230 g/kWHr. The ALH engine (upper left), while capable of a lower BSFC typically runs at a BSFC of about 250 g/kWHr - i.e. highway cruise at 2,200 rpm.

Crunching these numbers you get:
Prius thermal efficiency - 34.2% (230 BSFC)
ALH thermal efficiency - 34.5% (250 BSFC)

But then......the Prius is handicapped because its Atkinson cycle engine has to charge a battery pack with a charging efficiency of (so I've read) about 78%. And then drive an electric motor with an additional efficiency loss. So the diesel engine pulls ahead. And you can see why Prius owners are often disappointed with their fuel consumption. And why the Prius is mostly a fashion statement.

Here's the big secret. To get good fuel consumption you need a small engine - gasoline or diesel - operating at high rpm (low BSFC), coupled to a manual transmission in a lightweight vehicle.

Awesome, Thanks! that was a very informative post.

migbro said:

But then......the Prius is handicapped because its Atkinson cycle engine has to charge a battery pack with a charging efficiency of (so I've read) about 78%. And then drive an electric motor with an additional efficiency loss. So the diesel engine pulls ahead. And you can see why Prius owners are often disappointed with their fuel consumption. And why the Prius is mostly a fashion statement.

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FWIW the Prius HSD does not "have to" charge the battery pack. When loading on the engine is light (such as when decelerating / down hill runs) it can use the generator to increase load on the engine to keep it out of the inefficient areas of the map and store the energy generated into the battery along with that from regenerative braking. When loading is heavy (accelerating / up hill climbs), it supplements drive power with the electric motor to avoid that inefficient area of the bsfc map. This is a net increase in overall efficiency. For steady state cruising, there is little net current flow into or out of the battery. And as there are no clutches, and a minimal number of rotating parts, final drive losses are at least as good as a manual transmission, and most probably better. ORNL measured 500-1000W loss from 2000 to 3500rpms, or somewhere in the neighborhood of 1-3% transmission loss at highway speeds.

I'm not sure why you think Prius owners are disappointed, but with averages on fuelly running 47-48mpg for the Gen3s (2010-current) and 44-45mpg for the Gen 2s (2004-2009) it doesn't seem like they have much to be sad about

An engine with a TURBO has a higher efficiency - PERIOD!!!

The kind of people who post on fuely are often the kind of people who work at getting good mileage. Most people have no real idea what they're mileage is and say things like "around 50" until you ride 300 miles with them and calculate an honest 40.

I always find the subject of thermal efficiencies as very interesting. I think that the key to innovations that will spur more efficient engines in cars or heating/air conditioning systems for homes is thinking outside of the box. I have worked for many years in the electric generation industry. For power generation, thermal efficiencies around 50% are excellent; the newer gas turbine, combined cycle plants that have large gas turbines that exhaust into a heat steam recovery boiler that produces steam which is fed to a conventional steam turbine have thermal efficiencies around 50%. The company that I worked for (NRG) was looking to roll out a generator driven by a Stirling engine (a Stirling engine can run off of anything where you have a decent difference in temperature). I was curious how the thermal efficiency of this unit would compare to a typical gas engine home generator but my Company didn't want to release any technical details (anytime this happens I have serious doubts as many things get marketed on hype and not merit). I guess that one advantage of a Stirling Engine compared to a gas or diesel engine is that they are quiet. If you look at your typical gas or diesel engine in a vehicle; the two major areas where heat is wasted is in the exhaust (If you look at the >95% efficient home furnaces versus your older 60% efficient furnaces the gain in efficiency was captured by getting every last BTU of energy out of the flue gas prior to exhausting it) and the coolant via the radiator. If that heat could be used rather than wasted the overall efficiency could be boosted. The hybrid makes a lot of sense because it allows a smaller engine that operates in its most efficient zone to charge batteries when it has excess power to spare & then use it when needed. The regenerative braking captures some energy that would be wasted also. Anything that is going to be done can't make things to complex, too much complexity makes things unreliable.

Worth noting that the Prius engine only simulates the Atkinson cycle by holding the intake valve open past BDC, allowing some of the air to be forced back into the intake track. The actual Atkinson design was far more Rube Goldberg. Here's a nifty animation of a motor using the original Atkinson cycle (or near enough).

http://www.animatedengines.com/atkinson.html

Fun fact: Atkinson's design was in large part intended to avoid infringing on Otto's patents.

The history of the internal combustion engine is fascinating. Spent a good half hour going down the Wikipedia rabbit hole.

Jim

curtludwig said:

The kind of people who post on fuely are often the kind of people who work at getting good mileage. Most people have no real idea what they're mileage is and say things like "around 50" until you ride 300 miles with them and calculate an honest 40.

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So you're saying the similar number of people posting their TDI mileages on fuely are also the kind of people who work hard to get the 39-42mpg averages by year in their Golfs and Jettas?

I have a feeling there are quite a few people on this forum that do better than that, and don't find the fuely crowd very exceptional. IMHO the averages on fuely are pretty run of the mill, and given that they are tracked by tank tend to be reasonably accurate.

I average 52mpg lifetime on my '05 Prius, but I do work reasonably hard at it within the confines of not interfering with traffic and just needing to get where I'm going often. When I can hypermile my 12 mile rt commute I can do better than 65mpg, but its not practical most of the time. My wife has a lifetime average of 47mpg on her '08, because frankly she doesn't give a hoot

It seems kind of silly the way the Prius and TDI crowds go at each other, they are both a lot better than what most people are driving in my book.

Wow I had to watch the video for 5 minutes before I noticed the difference between the atkinson cycle and otto cycle.

JLMurphy said:

Worth noting that the Prius engine only simulates the Atkinson cycle by holding the intake valve open past BDC, allowing some of the air to be forced back into the intake track. The actual Atkinson design was far more Rube Goldberg. Here's a nifty animation of a motor using the original Atkinson cycle (or near enough).

http://www.animatedengines.com/atkinson.html

Fun fact: Atkinson's design was in large part intended to avoid infringing on Otto's patents.

The history of the internal combustion engine is fascinating. Spent a good half hour going down the Wikipedia rabbit hole.

Jim

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The crankshaft is offset in the Prius engine, too.
http://green.autoblog.com/2008/09/10/toyota-gets-all-techy-on-us-explains-atkinson-cycle/