Clett

Been thinking about this one for a while now. At the moment the best petrol engine efficiency you can get is in the Prius - 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 Prius-like 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?

ALi-B

iTrader: (1)

Well, certain tanks can run on both petrol and diesel. But 1mpg (or less) is not much to shout home about (or too loudly at the tax payer)

Petrol engines are moving ever closer to that of diesels (and vica versa) with the use of direct injection.

The limit factor of petrol is ignition control (octane) high compression with low octane fuel = detonation, thus retard ignition = less efficiency. So it wouldn't surprise me if they did away with ignition altogether.

BMW have addressed the issue of throttle restriction with their valvetronic system (no throttle).

Mazda used a Miller cycle engine on one of their v6 engines using a supercharger. Although efficient, it gave no success (probably something to do with it being a 2.3 when in reality it was a 3 litre engine and the cost of manufacture).

The problem with gaining extra high efficiency is that sometimes you create more in emissions in doing so. Think about it, modern cars are less efficient than they potentially could have been 14 years ago. Why? Weight being one, but also restriction via catalysts, EGR, air injection etc which reduces the power out of an engine and so affects the fuel consumption.

IMO petrol and diesel as fuel has had it's day, and through the arrogance and greed by the oil companies, technology has not been able to move forward as it should as the fuel is restricting what potentially could be done in improving the efficiency of combustion engines.

brickboy

Saab recently was trialling a turbocharged variable-compression engine -- it has an ECU-controlled hydraulic lifter which slightly tilts the cylinder head according to running conditions to raise or lower the compression ratio as needed. I think it's more for emissions and off-boost performance than anything radical though.

Honda also built a couple of fuel-injected 2-stroke bikes with what they called "Active Radical Combustion" -- basically controlled detonation of the mixture to give a more complete burn and lower emissions at low / medium rpm. Compression-ignition of the mixture at low throttle & low rpm, with the conventional ignition system taking over at higher rpm. These bikes actually did very well in a couple of enduro events in the US, but Honda doesn't seem to have developed the idea.

http://www.motorcycle.com/mo/mchonda/exp2_tech.html

what would scooby do

IIRC haven't Lotus got a patent for engine design that was plugless ??

Mungo

Petrol and diesel engines run completely differently, although I can see where you're coming from.
The idea with a petrol engine is to avoid knock - ignition of the mixture before TDC. This means having a fuel which can be compressed without igniting, and being able to delay the ignition at higher revs by retarding the spark. The lack of efficiency of petrol engines is due to not having the fuel mixture burnt fully during the combustion phase. There are various ways at improving this - combustion chamber design, twin spark plugs, etc. Most of the new engine designs are working on this, particularly the direct injection concept which does really borrow from diesel thinking - having the fuel in an extremely fine mist blown in exactly the right place in the chamber. Most petrol engines run at a compression ration of about 8:1 - any higher and you'd get knock.
Automotive diesels run at much higher comp. ratios - about 20:1, although I read that (I think) VW are now looking at 16:1. The diesel fuel is designed to explode on compression, which is why it's hard to have high-revving diesels - it's not easy to vary the compression ratio with engine speed. Developments are being made on this front. Modern diesels all have very high pressure direct injection - the fuel is highly pressurised as it is injected into the cyclinder which improves the misting and ensures as much of the fuel is burnt as possible. The reason old diesels puke out black smoke is because this used to be hard to control, so they were effectively over-fuelled most of the time.

If someone can remember more of their degree course than me, please correct the above...

John STI 8

I don't think Lotus own a patent, but they did do a lot of work on Gasoline HCCI (Homogenous Charge Compression Ignition) and CAI (Controlled Auto-Ignition) a couple of years ago. Honda, IFP (Institute France du Petrole) and a couple of universities were (and still are) at the forefront of the research.

J

Riprock

Doesnt the new accord diesel engine run at 16:1 or something around that point?Also most petrol engines ive seen tend to be around the 9 or 10 to 1.........going as High as 11:1 on the integra type R.But that might be just me

Dracoro

s2000 is 11:1 too so wouldn't surprise me if yer type r is the same or close.

ALi-B

iTrader: (1)

Jaguar were running V12 engines at 14.1:1 when they were working on their HE heads back in the early 80's

This is with mechanical controlled ignition and open loop EFi....I'm amazed it never melted every piston. But then they did have lovely leaded 5 star

(prduction Jag v12's ran at a more modest 12.5:1 - beat that!! )

carl

Mitsubishi's GDI (Gasoline Direct Injection) was as efficient as a diesel (mid 40s MPG) but never really took off

Paul_M

GDI is still doing the rounds, and has been adopted by other manufacturers under various guises, e.g some of the Volvo S40s and even the new Mondeo has a GDI 1.8 available. It's still pretty overshadowed by diesels for company car tax hence why it hasn't really taken off.

BTW the BMW valvetronic does away with the throttle but it's still effectively throttling the air volume by controlling the lift and duration of the valve opening, thus while it's better than the conventional "one throttle for all" approach it's still restricting airflow unlike a diesel.

NotoriousREV

In a similar vein, why don't we see small 2 stroke diesels? They're much more efficient than 4 strokes with the potential of producing much more power and torque.

carl

Not really. Unless they're EEC Type IV (or something like that), diesels get a 3% company car tax loading. So a GDI with similar emissions and MPG would be better from a tax standpoint.

icantthinkofone

How are you measuring efficiency? There's always the small matter that diesel oil has a higher calorific value - it contains more energy per litre than petrol.......

Clett

Yup - a gallon of diesel contains about 18% more chemical energy than a gallon of petrol (depending on where you are in the world and the time of year). What I mean by efficiency is with this accounted for ie chemical energy in divided by mechanical energy out at the flywheel.

Here's what I'm thinking so far:

There is no reason why you couldn't have a compression ignition petrol engine. The reason it hasn't been done so far is because avoiding detonation (knocking, pinging, whatever else you want to call it) has been the major aim all along simply because it is quieter and smoother to use spark ignition and stochiometry etc. Who would want a petrol engine that sounded like an old diesel?

But, with todays multiple-injection TDi, the knocking is much less of a problem - and compression ignition engines are as a result pretty smooth and quiet these days.

As for the efficiency thing, I reckon the major gains are in the expansion stroke. The greater the expansion, the more energy you are extracting out of the fuel. So, I think Atkinson cycle compression ignition petrol could well be the answer for best efficiency.

Steve Wilson

If I remember correctly Honda produced an engine that ran partially on compression ignition some years ago. It was on a trail bike called a 250 CRM AR (Active Radicle).

It was a two stroke and I think it was designed to use some of the exhaust charge from the previous cycle to ignite the current charge with compression rather than an electrical spark. (It did use electrical spark some of the time).

The idea was to reduce emissions which are notoriously bad on two strokes.

I believe that the AR engine was more tourquey than the conventional engine but was harder to tune and would not produce as much peak power.

I think there are some of these bikes still around. Mugen used to produce pipes for them.

Mungo

Possibly the difficulty in tuning and the lack of peak power was due to an inability to rev? i.e. the age-old problem associated with compression ignition. A great wedge of torque but only over a fairly limited rev range.