[clett]

Been thinking about this one for a while now. Once plug-in hybrids come on line, engine torque and driveability will be much less significant, as the electric buffer will be so big the only thing that will be wanted from the engine will be as high outright thermal efficiency as possible.

At the moment the best petrol engine efficiency you can get is about 35% under ideal conditions. However, VW's best TDi engines are now at about 46% efficient. This means that drop for drop, the TDi engine can allow a vehicle to travel 31% further (ie it is able to extract 31% more energy out of every drop of fuel than a spark ignition engine).

As I see it, most of this improvement is down to lack of throttle, and the diesel engine's high compression ratio (20-24:1). Surely, therefore, it would be trivial to make a petrol fuelled compression ignition engine? I read once that old Peugeot diesels were meant to take up to 25% petrol in their diesel tanks during the winter - and on the farm I have seen in the past diesel engines run (accidentally) on about 50% petrol with no ill effects at all (other than much easier starting).

Is there some reason why spark ignition must be used with petrol? Certainly stochiometry must be maintained in these engines (ie amount of air and fuel must match), but this is not the case with diesel engines. In fact, petrol engine efficiency plummets when not used at full throttle, and this loss isn't nearly as pronounced in diesel engines.

Final question - what are peoples opinions on the feasibility of an Atkinson (or Miller) cycle diesel engine? Would likely not be able to rev more than about 3,000rpm, but a potential expansion ratio of 30:1 could mean an efficiency maybe in the 60%s?

 

[Yves M.]

Hi,

The atkinson engine got me this idea. In order to help the gas engine efficiency, I would suggest to remove the throtle plate that causes the engine to loose energy trying to pump air out or the reduced intake when the throlte is not wide open. Replace it with intake valve timming control.

In oder to do this, I would sugest to control the intake valve through a separate intake and exhaust camshaft. The intake camshaft could be rotated by the degrees necessary to control the engine speed:
If the timming is retarded after TDC, the engine speed is lower. As you increase the timming to TDC and before TDC than the engine will be faster (like engines now)

To control the timming, the use of a timming chain with enough looseness and 2 idle pully (one on each side of the chain, before and after the camshaft). If the pullys are moved on one side it increases the valve timming and if moved to the other side is decreases the timming. Both pully work together to keep the chain tight which ever side the looseness of the chain is.

As you tighten the chain, it turns the camshaft position in relation to the crankshaft. It is directly in relation to the length of the chain that is removed between the camshaft and crankshaft.

Now I do not know how precise the timming needs to be to allow the timming to control the engine speed. Would it be hard to stay at idle? Don't know.

[I moved the issue to the General forum, the news forums did not seam to be the correct place]

 

[Foxpaw]

I can't say for sure, but I'll tell you what I do know about gasoline and diesel being used in each other's engines.

If you put gasoline in a diesel engine, you risk it "running away." Basically, the gasoline burns so much faster the engine keeps accelerating and accelerating until it explodes. Not sure what ratio of gasoline is required for this to happen, or if it happens to all diesel engines, but apparently, it happens to some.

Gasoline engines can suffer from a condition known as "dieseling." This is when the gasoline is being ignited from pure compression as you've described. It's usually a symptom of an engine in very rough shape. (It's the knocking sound that goes on for a few seconds when you shut down some engines) Not sure if this affects vehicles with an electric fuel pump or not, however. I'm not sure why it is, but to my understanding, dieseling in this manner is extremely hard on the engine.

 

[jtmco]

As you stated, the diesel is more efficient because of the higher compression ratio. If a diesel and gasoline engine were built with the exact same compression ratio, the gasoline engine would actually be more efficient.

Of course, gasoline engines are limited in compression ratio by knock or autoignition. To combat this, gasoline is formulated to resist autoignition. The higher the octane rating, the less likely gasoline is to ignite purely because of compression. Of course, this is exactly what you want in a diesel engine. So while including some percentage of gasoline in the diesel fuel might be tolerated, adding too much will simply make the fuel difficult to ignite and you will have incomplete combustion in the cylinders. This would lead to increased hydrocarbon emissions among other things.

I don't know enough about the Atkinson and Miller Cycles to comment on your other question. I understand the Prius uses an Atkinson Cycle and some Mazdas have used a Miller Cycle.

Sorry if I'm stating what everyone already knows - I spend all day teaching and sometimes it is hard to get it out of my system.

 

[clett]

Hi,

The atkinson engine got me this idea. In order to help the gas engine efficiency, I would suggest to remove the throtle plate that causes the engine to loose energy trying to pump air out or the reduced intake when the throlte is not wide open. Replace it with intake valve timming control.

In oder to do this, I would sugest to control the intake valve through a separate intake and exhaust camshaft. The intake camshaft could be rotated by the degrees necessary to control the engine speed:
If the timming is retarded after TDC, the engine speed is lower. As you increase the timming to TDC and before TDC than the engine will be faster (like engines now)

To control the timming, the use of a timming chain with enough looseness and 2 idle pully (one on each side of the chain, before and after the camshaft). If the pullys are moved on one side it increases the valve timming and if moved to the other side is decreases the timming. Both pully work together to keep the chain tight which ever side the looseness of the chain is.

As you tighten the chain, it turns the camshaft position in relation to the crankshaft. It is directly in relation to the length of the chain that is removed between the camshaft and crankshaft.

Now I do not know how precise the timming needs to be to allow the timming to control the engine speed. Would it be hard to stay at idle? Don't know.

[I moved the issue to the General forum, the news forums did not seam to be the correct place]

Hi Yves,

Good idea! Unfortunately it has already been nabbed by the Germans! BMW have a petrol engine without a throttle (see here for some nice diagrams), and the mechanism for continually varying valve timing is similar to what you've described (see here).

Both still spark ignition engines though with compression ratios about half that of a diesel engine!

Clett

 

[Yves M.]

That's to bad, I thought I had a good idea and I was first.

 

[Armin]

I would suggest to remove the throtle plate that causes the engine to loose energy trying to pump air out or the reduced intake when the throlte is not wide open. Replace it with intake valve timming control.

Yves,

help me understand how this reduces the throtteling losses. The way I look at it, the problem is that the engine is drawing air through a restriction, causing losses. How does it matter if that restriction is a throttle plate or a shorter valve opening time?

If I remember correctly, the problem with gasoline is that it needs stochiometric combustion (fixed fuel/air ratio) to burn correctly. Diesel doesn't. So you can have a cylinder full of air with very little diesel in it. But you can't do that with gasoline. Ergo, you need to limit the airflow coming in and whatever way you do that causes throtteling losses, no?

 

[clett]

If I remember correctly, the problem with gasoline is that it needs stochiometric combustion (fixed fuel/air ratio) to burn correctly.

That's true for the way petrol engines are built today (ie spark ignition). If it runs too lean, it burns too hot and you can melt your pistons. However, suppose that as with today's latest diesel engines you only injected fuel directly into the cylinder at or around the moments of highest compression. To reduce knock and give a smoother delivery, modern diesels do this in up to 5 separate mini-injections per cycle, with little pre-injections to smooth the whole process out. So surely if these were optimised accordingly, using a different fuel wouldn't make a huge difference? OK, the flame front is faster with petrol, but surely the principle is the same? I guess the question I'm really asking is; does the compression ignition principle mean that stochiometry can be avoided for other fuels - or is it specific to diesel?

 

[Yves M.]

Hi Armin (I'll try to be as clear as possible but it is not clear in my mind)

As I read from Clett this tread about gas engine efficiency and Atkinson cycle, I associated in a similar way, the Toyota uses a false Atkinson engine, it could control the engine speed.

The Atkinson uses levers for the piston to have different travel length. The 'false' Atkinson uses the valve timming to simulate the Atkinson cycle. Toyota uses valve timming to release some air mixture into the intake to reduce the incomming mixture at compression cycle. This reduces at the same time the pumping loss.

This is where I thought that if one could control even more the intake valve timming, this could prove to control the engine speed. So there would not need a throtle plate. To reduce the pumping loss. Of course, as for the Toyota false Atkinson, the air comming into the piston chamber is put back into the intake.

For example, at idle almost all of the air would be removed out of the cylinder and would be freely going in/out of the different cylinders. The advantage here is that the pistons would be moving with no vacum into the intake and no pressure into the compression stroke.

What I would like to know is (as for the Toyota engine) what happens is there is a back fire. Does the intake blow out and is there damage to the other engine parts.

Clett, I suppose that it would be possible to create an Atkinson diesel. I would suspect that such an engine would not have a high top revolution (RPM) and would be best suited for stationnary engines like in big boats or generators
It could be suited to provide electricity for long range of an 100% electric car.

And you have a good idea about the gas injection.

 

[james]

I think - well, call it an educated guess - that the drawback would be a very extreme knock. The point is that you don't want gasoline (or diesel) to explode, you want rapid, smooth combustion.

However, if it's fuel efficiency that you want, I seem to recall that a gas turbine has a higher theoretic efficiency than any piston engine. So you hook a small gas turbine to a generator, and use the hybrid battery system as transmission/torque converter...

 

[Corey872]

Foxpaw Wrote:

I can't say for sure, but I'll tell you what I do know about gasoline and diesel being used in each other's engines.

If you put gasoline in a diesel engine, you risk it "running away."

Were there some specific or unusual conditions surrounding these incidents? From the incidents that I have witnessed, just the opposite was true. Diesel fuel is specifically "designed" to be ignited by compression alone. Gasoline is specifically "designed" to resist attempts to ignite on compression alone and only ignite once a high voltage spark is introduced in the combusion chamber.

The gas-in-diesel will certainly reduce the lubricating properties of the diesel and may ruin the injectors or injection pump, but in four out of four instances I have personally witnessed (not my fault!) the diesel engine simply lost power and died. I have also witnessed an instance where isopropyl alcohol was put in a diesel fuel tank (again, not my fault!)...strangely enough, the "raw" alcohol had a straw-yellow color just like the diesel and a specific gravity which was ~35 API, right in line with diesel, too. Again, due to the inability of the alcohol to ignite under compression, the diesel engines (yes, more than one) simply shut down.

Lastly, (this was my doing) several years ago, I converted an old diesel truck with a blown engine over to a gasoline engine. When I was done, I didn't have any way to get rid of the ~3 gal of diesel drained from the tank, so I simply dumped it back in with ~30 gallons of gasoline. The result was a substantial amount of "pinging" when the engine was under any type of load. This cleared up on the next tank fill when the diesel was gone.

I can only speculate that the compression ignition properties of the diesel fuel were still active even though it was mixed with the gasoline. Once the spark lit the gasoline, cylinder pressures would have risen rapidly and at some point the unburned gas-diesel mix (away from the spark plug) must have had sufficient compression to light off on its own. The two resulting flame fronts would have accounted for the "ping"