A&Q about 350Z
Q:
It's a well known fact that a smaller engine will be more efficient when it's used in car. This is based on the simple fact that the otto engine is more efficient at higher loads than on part load. The efficiency of an engine at full load is typically around 30% or so, at very low loads the efficiency can be as low as 10%.
It's also well known that most car engines spend most of their time running at very low loads since only a small amount of power (5-20kW is enough for most cases) is needed to keep a car at a constant speed. So by using a smaller engine engine load can be kept much higher and the fuel consumption can be reduced.
With the smaller engine funning at higher load friction losses will be smaller, the heatlosses will be smaller, the pumping losses will be smaller and so on.
When it comes to estimation of heat losses and friction there are some softwares that can calculate those with quite good results, these softwares are commonly used by automotive engineers. Lotus Engine Simulation is such a software, and there's a single cylinder freeware availible for download on
As for diesel engines and their compression ratios, the use of a high compression ratio is needed if a compression ignited engine should be able to run and especially to cold start when it's cold outside.
A high compression ratio, and thereby a high expansion ratio means a higher thermal efficiency. But high compression ratios also means high cylinder pressures. High cylinder pressures means that the engine must be structurally stronger (heavier), it also means that the friction losses increase (piston ring friction). Since friction increase with cylinder pressures, mechanical efficiency goes down when cylinder pressures become really high. Because of that reason a compression ratio should be chosen so:
1. The engine easily can be cold started
2. That the thermal + mechanical efficiency is high
A very simple way to estimate the friction losses is by:
FMEP = 0.3 + 0.04*(vP)^1.3
where
vP = piston velocity in m/s
FMEP = Friction Mean Effective Pressure in bar
FMEP is the difference between BMEP and IMEP:
IMEP - BMEP = FMEP
where
IMEP = Indicated Mean Effective Pressure
BMEP = Brake Mean Effective Pressure
Engine power, power lost though friction or engine power with no friction losses can be calculated
P [kW] = BMEP [bar] x V [litres] x n [rpm] / 1200
P [kW] = FMEP [bar] x V [litres] x n [rpm] / 1200
P [kW] = IMEP [bar] x V [litres] x n [rpm] / 1200
A:
^ i like this guy
And no offense, but i trust Kiwi's knowledge, experience and intuition more than someone who doesnt have a complete understanding of what's going on. I'm sure he busted his rump in school to get his engineering degree, which isn't easy i know, and the fact that he even finished it says to me that he isn't bullshitting with this stuff. He probably comes off a little testy because of people that keep feeding him pipe dream engineering projects and then get all butthurt when he shoots down their dreams with facts and experience.
And the only reason the Corvettes are pulling 28 mpg on the highway (and ONLY on the highway) is because their overdrive is so insanely tall that the engine is just above idle at 65-70mph, and its aerodynamics mean that it doesnt need THAT much torque to keep it going. If you actually drive it 'normally' it wouldnt get near 28 mpg.
A:
what do you do at that chevy dealership?
A:
Car sales. It is my job to know the cars stats off the top of my head. I used to be a mechanic but that really sucks sometimes, so I got into sales. I think we all know that engines are most effecent when they are put under heavier loads, that is why in my concept engine it is designed to opperate at such extreamly low RPMs, the higher thermal effecency of a larger bore, and the load percentage of a small 4 cylinder. I am not for sure that I would totaly surpass the most effecent four bangers out there, but I know I could get close with a very powerfull, very exciting engine, Just imagine Mclaren F1 power and Toyota Yaris ecconomy, and the really great thing about it is that it would not cost a fortune to produce. No need for titanium valve springs, pistons and rods balanced to .5g, it's power comes from it's unbeatable torque not from a 9000RPM readline.
A:
as i see it, you (the thread starter) are talking about a slow revving (1000 tops) engine with a long stroke and huge bore.
engines like this are currently being used in a much larger scale in ocan vessels.
i imagine that the queen mary and similar ship engines dont rev much past 700 rpm. ( i think the queen mary uses boilers, but W/E)
lower rpms are better anyway because you can get more acurate timing and can get away with less overlap and have the valves shut instantly, meaning less wasted energy.
as for huge engine bay problems, use an inline and mount it way low. put the accessories on the side. a cowl induction hood could add some clearance.
i'm not claiming to have any credentials or anything, but i do have logic, and a firm grasp of mechanical devices and how they work.
i would go ahead with the huge bore, but i think a short stroke would be more do-able.
the chevy 302 has a 4" bore and a 3" stroke. it has been taken past 10,000 rpms in racing applications.
i would keep stroke between 4 and 5 inches, but go crazy with the bore.
it would fit into an engine bay better.
i'd use diesel, and go crazy with compression.
a turbo can increase thermo efficiency, so it would be good to have too.
A:
Damn straight.
Most people also get testy when told their super-dooper perpetual motion machine is never going to work.
A:
You still don't quite get it, The stroke is not excessive in fact it is undersquare, the reason for this? To allow it to rev to 4000+RPM when you want excessive power
but being able to cruse at or below 500 RPM to maxamize fuel ecconomy. Direct injection and a very low idel speed will prevent the massive engine from burning excesive amounts of fuel when the vehicle is stoped.
A:
I have a suggestion:
kachok25, why don't you stop talking and start doing? Go to the drawing board and try to draw your engine first. I'm sure you'll realize you need to read something on the subject as a result. And when you read and understand everything that needs to be understood to build an engine, come back and re-read the thread. I bet you'll be surprised.
A:
I'm sure you can find a machine shop willing to do this. and i give another +1 towards Kiwibacon. you'd be surprised what you learn in university. if you took an engineering course you'd learn things that'll help you understand 1)your own ideas better and why they will or will not work and 2)what other people are telling you. it's a hell of alot easier to explain things to someone who has the base knowledge required.
And i know how tough engineering can be. i'm in first year engineering, easy for now, but once you hit second and subsequent years it gets intensive, you have to be smart to get through, which is why i'll take kiwi's professional engineering over your mechanics and car sales for discussions about theoretical engines and whatnot.
A:
Well put.
Even drawing the simple 2d layout of such an engine (crank, conrod, piston, head, valvetrain) would prove it unworkable.
That's without getting into the thermodynamics and mechanics of it.
A:
The efficiency of an idling engine is 0%.
Give an idling engine a small load and you efficiency might rise to 1-2%
A:
I've been doing engines for a living...
A:
Surely you mean 0.3-30mpg.
*edit for typing*
A:
If you don't have any usefull scientific information why don't you just go away, pointless unfounded bashing is pointless, if you don't think it will work give me a ratonal reason why and not some off the top of your head BS, I am not trying to be rude but I am tired of peoples crap, yeah I understand that this is not the normal way things are done that is why I am asking question and not telling everyone how it should be done. Even if I had the funds to build this right now I would still be checking out the mathamatics first, which is what I am doing now.
A:
The largest of the ship engines are turbocharged 2-stroke direct injection compression ignition engines running on fuel oil. One of the largest ship engines in this class is the Wärtsilä-Sulzer RT-flex96C. It has bore of 960 mm and a stroke of 2500 mm, it runs at 92-102 rpm and it's designed to operate at high loads.
As you can see it has a very long stroke, about 2.5 times the bore. This means that when the engine is running the piston mean velocity is around 8.5 m/s, similar to that of a car engine.
The engine isn't efficient because it's large, its size and power output is of benefit though. As we increase the size of an engine displacement increases faster than cylinder surface area, and if the power ouput increases proportionally against the displacement, heat losses decrease. Of the same reason a large displacement car engine which only needs to develop a certain power output the heat losses will increase, especially if we have an increase in bore rather than stroke.
The ship engine is also efficient because it's a two stroke, two stokes per cycle means that you only need half the engine speed for a certain power output and that equals less friction.
The engine has overlapping port/valve timing, naturally since it's a two stroke, but since it is direct injected no fuel is lost though the exhaust.
The engine is turbocharged, these turbochargers are used as scavenge pumps and force feed the engine with a high density charge. This increases power output while friction and heat losses remains small.
In a car an engine with a huge bore or stroke would not be efficient. It's much better to design a very small direct injected turbo engine, this concept would save you probably 10-20% fuel compared to a NA engine with similar power output.