[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]

Re: I5 revival

SOP8920@Siena.edu wrote:
> >There is already new I5 available in Europe, but it is a diesel. A
> >wonderfull engine, 5 cyl,
> >2.5L, turbo, 150 hp, TDI (direct injection). The first quattro diesel ever
> >made is also available, it is an A6.
> >Later,
> >V. Jokic
> >St. Catharines
> >Canada
> (1) Is this the same TDI that is available now in the Passat?
> (2) I was told, or read somewhere, that diesel engines are made more robust and
> last much longer than gasoline engines. Since I barely understand much about
> gasoline engines, I shrudder to ask. BUT.. if somebody could shed light on
> this. We used to own a MB 300TD. It started poor as all hell in the winter,
> and was slow as a dog. Yet.. the car fascinated me.
> I just don't understand the technology I guess. For instance, why is it better
> for truckers to run their diesel engines all night long and not shut them off?
> Yet, with gasoline engines... we don't exactly leave 'em parked in the driveway
> idling all night. What's going on here?
> Oh.. and what the heck is a glow-plug anyway?
>                                -Osman Parvez
>                                 89 200q
>                                 Albany NY

Diesels don't have spark plugs.  Fuel is injected into the cylinder and 
ignites as the result of essentially adiabatic heating of the intake 
charge as the piston compresses it.  In other words, diesels have 
MUCH higher compression ratios than Gasoline engines, in the range of 
30:1.  The fuel/air mixture is compressed to such an extent that the fuel 
spontaneously ignites.  PV=nRT. There is no such thing as "ignition 
timing" in a diesel as it is understood in a gasoline engine.  In a 
gasoline engine, spontaneous combustion is called "detonation", or knock. 
In a diesel, this is called "everything's fine".  A "glow plug" is a 
heating device (think of an immersion heater that you put into a bathtub) 
to warm the cylinders on cold days so that the starter doesn't have to 
work through too many cycles to get the cylinder warm enough for 
combustion to occur.

Here is where my knowledge is hazy:

Kerosene is a very smooth-burning fuel, and doesn't burn as quickly as 
gasoline, but it has the potential to generate more heat of combustion 
per gallon.  As a result of the lack of direct ignition and the 
aforementioned fact, a diesel engine has an upper rev limit that is 
determined by the amount of time it takes for the fuel to burn and apply 
combustion pressure to the pistons.  Of course, you can't rev a gasoline 
engine higher than the RPM that gives the fuel enough time to burn and 
overcome the frictional losses (if you can get it to survive that kind of 
reciprocating velocity in the first place, which Honda did with its 
four-stroke NR500, 20,000 RPM, oval-piston, 8-valve per cylinder GP 
racebike) of the engine either.

Am I right in remembering that diesels have a very broad torque range 
because they run at essentially wide-open-throttle (air restriction) all 
the time, and the only thing that varies is the fuel content?      

Also, most diesel truck motors have roller-bearing crankshafts and