97 EB: How can I improve gas mileage a little bit?

Sorry for the delay, Ben..... been at school again and polishing, polishing, polishing.... scooter weather is coming...

With the gas motor, once we are up to our cruise speed, we only need deliver enough throttle opening to sustain the RPM...... manifold vacuum usually returns to a fairly high (though not generally as high as idle or coastdown) value..... we are still "throttling" the intake air the cylinder cannot completely fill and our cam design cannot scavenge the cylinder well......

Our 20 cubic inch diesel will still have pretty close to 20 cubic inches of atmospheric air in it while our 20 cubic inch gas motor will likely have significantly less atmospheric air as well as a small amount of spent exhaust gasses. Throttling the intake is required for gas engines but is inefficient because of it's nature...... when the intake opens, the piston moves down against the intake vacuum on the gas motor whilst the psiton moves down against atmospheric in the diesel (normally aspirated, remember?) - we're not dealing with that Gawd-awful trap door in the intake stream...

Diesel engines never have "manifold vacuum" whilst gas motors almost always have some amount of manifold vacuum.

HTH

Jim Warman snipped-for-privacy@telusplanet.net

Vacuum pump..... the older ones used an engine driven pump while the new ones use an electric pump. Many used a "HydroBoost" type brake booster but most of these pumps produce enough to even run the brake booster.

Jim Warman snipped-for-privacy@telusplanet.net

Jim,

While a gasoline engine may have to expend a tiny bit more energy to pull air/fuel into it's cylinder than a diesel on the intake stroke, the diesel has to work a tiny bit harder forcefully injecting a fuel/air mixture into the cylinder that has already been fully compressed so maybe the two items balances each other out?

Ben >Sorry for the delay, Ben..... been at school again and polishing, polishing,

Actually, the pumping losses experienced any time manifold vacuum is high are much more than negligible. Vacuum is a very powerful force in nature and it can be quite easily misunderstood as to how powerful it really is (remember that 'absolute' vacuum is on the order of 30"of mercury at sea level.... memory don't fail me now...) and here we are driving a piston down 'against' 1/3 to 1/2 that.

The effort expended in injecting fuel into a diesel cylinder (static compression is 300 to 400 or so psi compared to 130 to 160 for gasoline) even at pressures like 3000 psi is nowhere comparable to wasted energy in a gas motor. Many of todays diesels feature HEUI injectors (hydraulicly electronically unit injector) and are probably one of the wonders of the modern world in light of the fuel control they offer. The high pressure oil pump that feeds these injectors offers up a much smaller parasitic loss under most operating conditions than the older high pressure fuel pumps. Let's not forget that we can also keep adding fuel as the piston travel down on power stroke..... in the gas motor, what the cylinder got squirted at the back of the intake valve is all the cylinder is gong to get.

Once we get our vehicle up to speed, it will likely only require 20 hp (and I'm using hp quite loosley here since it is indeed torque that is the measure of work)or so to maintain that speed on level gtound, yet we are spinning our motor merrily at an rpm where it produces 5 or even 10 times that amount of power. Todays technology allows the vehicle to lean the mixture out (limiting torque), but it can only go so far before we run into serious engine damage and/or increased tailpipe emissions. The diesel doesn't have that limitation..... unthrottled, we control torque output by varying the amount of fuel added to the cylinder - at cruise, we only add enough fuel to remain at cruise without incurring any lean mixture woes.

In spite of it's poor extreme cold weather starting characteristics, noise, smell and weight..... the diesel remains to be a more efficient use of the Otto-cycle principal than it's gasoline fueled cousin.

Regards

Jim Warman snipped-for-privacy@telusplanet.net

Back in the 70's, several GMC/Chevy models used the power steering pump to provide power for the brakes.

I think you've got this backwards.

Torque is a measure of twisting force, while work is the transfer of energy.

Torque applied to an axle that doesn't turn is still applied, even though it does no work.

Yep.... After looking at my message I see oldtimers disease... the "HydroBoost" was the oldframe maounted vacuum booster we used to see on larger trucks with hydraulic brakes. IIRC, GM called their hydraulic booster a "Hydraboost" which does use power steering pressure for the unit. Both Ford and Dodge have used this setup....

I still have trouble with the concept some days..... yes - torque is the force and hp is the "rate of doing work".... scratch out my parenthesis and my statement reads better (two or eight beverages can do that to a man).