Date: Sun, 24 Feb 2008 19:39:31 -0500
Reply-To: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Subject: Re: A Comment on TDi Engine rpm vs. Longevity and Economy
In-Reply-To: <d67.25939c6a.34f2bc0d@aol.com>
Content-Type: text/plain; charset="iso-8859-1"
You forgot to mention that as you lower the engine speed increasing the
torque load, chamber pressures, and combustion temps, you are also
increasing the production of NOx. NOx is one of the pollutants making it
difficult for new Diesels to get compliance.
Dennis.
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Frank Grunthaner
Sent: Sunday, February 24, 2008 7:25 AM
To: vanagon@GERRY.VANAGON.COM
Subject: A Comment on TDi Engine rpm vs. Longevity and Economy
For some time I have seen the traffic on Vanagon TDi related lists as
individuals assume elevated rpms are deleterious to both engine longevity
and
effective fuel economy. New converters are advised to regear transmissions
dropping
3rd and 4th to put rpms in the range of 3000 for 70 mph.
I have tried to address this issue in the past (see Vanagon archives),
wherein I offered a series of tables and measurements (for gasoline
powered engines)
showing that several key points dominate the reality of this situation.
For
this note, let me review the key points as they apply to TDi engines:
1. For a given speed, on a level highway the power required to maintain
that speed is a function of the mass, the aerodynamic drag and powertrain
losses.
Not a function of engine rpm. The thrust required to maintain that speed
is a
constant at the tire/road contact patch and is a function of engine rpm.
2. For a given mass and Vanagon type (Westy, passenger van, panel van,
pickup
or syncro) the responsiveness or ability to accelerate from a reference
speed
is directly related to the torque reserve available at the tire/road
contact
patch. Reserve means how much power is left after providing the thrust to
maintain the vehicle at the starting velocity. This torque is a product of
the net
gear ratio from the transmission, through the final drive and finally
through
the lever arm operating from the axle centerline to the tire/road contact
patch. Larger tire (fewer revs per mile) but same gear ratio and final
drive
requires more torque from the engine to maintain a given amount of thrust.
3. Stress on engine components is directly related to peak chamber
pressures
(which are in turn directly related to cylinder temperatures). The higher
the
load factor at any given rpm, the higher the pressure. The more torque
required by the driver at any given rpm, the higher the load factor and
the higher
the chamber pressure. Direct measurements on the 1Z/AHU and the ALH 1.9L
TDi
engines by VW engineers show that for the same load (torque output) at
3000 rpm
and 4500 rpm, the 4500 rpm case has a 1.1% higher chamber pressure. In
other
words, stress in the VW TDi 1.9L engine is only a function of torque
output, not
engine speed.
4. The real fuel consumption of the engine is given by the product of
instantaneous power output required and the brake specific fuel
consumption
(efficiency) of the engine at the speed and load factor the driver
demands. Figures are
published for the BSFC for the stock engines at maximum load. The units
are
grams of fuel at 20C per kilowatt hour of power delivered. For the 1Z/AHU
with
traditional wastegated turbocharger, the numbers are: 203@2100, 220@3000,
222@3500, 235@4000 and 250@4500. For the 1Z/AHU with VNT turbo or the ALH
engine,
the numbers are: 196@1750, 214@3000, 217@3500, 225@4000 and 241@4500.
Again
these are the full load at any given rpm values. The consumption at less
than
full load is different than these numbers but never better than the
optimal
efficiency number (2100 for the 1Z and 1750 for the ALH) due to the fluid
dynamic
efficiency variations.
5. The so-called redline for these engines is a mechanical fluid flow
limit
related to the diesel fuel burn time. A given amount of injected fuel
takes a
well-known time to burn. If the burn has not completed by the time the
exhaust
valve opens, the fuel will complete its burn in the exhaust tract, quickly
destroying the turbocharger. As engine speed increases the time available
to
complete the burn gets shorter. On the 1Z/AHU and ALH engines, the stock
controller shuts the fuel delivery off at 5200 rpm. This is the ECU or
mechanical pump
enforced redline. These engines have an engine destruction onset redline
(compare directly to gasoline engine redlines) of between 7000 and 7800
rpm. When
TDi bottom ends with rods, similar composition pistons and rings are used
in
racing gasoline engines they typically break above 10,000 rpm. In other
words,
the bottom ends of these engines are impressive and highly overbuilt for
the
diesel application.
6. The torque curve on the stock engine begins a sharp decline around 4200
rpm with a horsepower falloff at 4400 rpm. This is largely a function of
factory
tuning parameters. With a readily available chip (see Rocketchip.com for
example), these engines experience the torque drop-off at about 4500 rpm
and the
power drop-off at 4800 - 5000 rpm.
7. The modern VW diesel engine is designed for continuous high speed
service.
It is rated for maximum horsepower at 4000 rpm. In the design stage as
well
as quality assurance testing the engines are run for the equivalent of
200,000
miles with engine speeds randomly varying followed by continuous 20 hour
steady speed cycles. These engines are continuously run at 4500 rpm.
So what does this all come down to? Well, in a list of points in no
particular order:
a.) If you regear your transmission from 4000 rpm at your desired cruising
speed to 3000, you will save between 5 and 6% of your fuel cost due that
rpm
change. You may see more, but that is strictly due to changed driving
habits. If
the engine has a lower torque reserve for acceleration or hill climbing
you
may well back-off to enjoy the scenery. You will increase the chamber
pressure
in the engine by 33% or more if thermal effects are included. This means
higher
bearing loads, thinner oil lubricating films, and higher piston head and
exhaust valve temperatures. This means a reduced engine life compared to
the 4000
rpm case. You will also piss away a significant number of bones on
transmission work.
b.) Regearing from 4000 to 3500 is less of a fuel saving and less stress
difference, but the direction of effects is the same.
c.) Regearing from a 4500 rpm cruising speed to a 3000 rpm value will give
a
10 to 12% fuel savings. Now the stress factor has increased by 50%. All
other
comments of a.) still apply. The change from 4500 to 3500 is left as an
exercise for the reader.
d.) It should be evident that a reasonably maintained engine in stock tune
can be run at 4200 to 44 rpm all day with no longevity ramifications. With
a VNT
turbo and a chip, 4500 to 5000 are accessible Cruise up to 4500 and
emergency
use to 5000.
e.) Vibration and noise are a problem, but as I have indicated in the
past, a
snorkel to off-road air cleaner with air intake through the sealed driver
side compartment, 2.5 inch exhaust and pneumatic motor mounts eliminate
this
problem. A 500+ watt stereo system with satellite and HD radio together
with the
ipod can contribute sufficient decibels of Mozart to eliminate the issue.
f.) In my install (1Z engine, VNT turbo, PD150 intake with no EGR,
Rocketchip, PP520 nozzles, lift pump, windage tray and oil pan baffle, DK
transmission
with 4 spider carrier, SA splash plates and 215/75-15 tires), my over the
ground speeds are 62 @3000, 72 @3500, 82@4000 and 92@4500 in 4th. In 3rd,
the
numbers are 43@3000, 50@3500, 57@4000 and 65@4500. The shift in LA traffic
mergers
is typically @4000 to 4500 for and easy pickup by 4th. Just a stock DK
with
strengthened pinion. I'm considering going back down to 27-8.50-14 tires
which
would give me a 75 mph cruising speed of 3750 rpm, and 90 mph @4500.
I rest assured that immediately following this post we will find the
conventional wisdom dispensed to the effect that a regear to 3000 is
essential for the
TDi Vanagon combination. Humbug!
BTW, for those who are searching for a conversion that puts serious vim
and
vinegar in the manual trans Vanagon for modest investment, easy service
and
serious longevity without spending the cash on the TDi route, the pre-ABA
engines
including the Digifant RV and the Audi 3A tied up to the DZ transmission
are
an impressive combination. Highly recommended!
As always, my apologies for the length.
Frank Grunthaner
In a message dated 2/23/08 11:12:37 PM, ScottDaniel@turbovans.com writes:
> 4,000 rpm in a TD engine is 'thrashing' a bit more I think, that a short
> stroke waterboxer is.
>
> 3,500 rpm in a diesel is as low as I would want for my highest cruising
> speed that I would expect to do without undo stress.
>
> I wouldn't ask it to do 70 at 3,500....... I wouldn't ask it to do 70
> stressful on the engine
>
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