Driveability Corner The last column on fuel delivery calculations generated a number of questions, especially about volumetric efficiency. Now is a good time to revisit VE, in case you missed Mark’s earlier columns.

hat is volumetric effi- gine) factors is quite a bit longer: air induc- ciency? If you crank a tion flow/restriction, throttle, exhaust flow/re- 1-liter engine over striction, valve timing, valve opening, EGR, two revolutions, under positive aspiration (supercharging and tur- ideal conditions, it bocharging), variable-displacement engines Mark would pump 1 liter of (cylinder disabling) and induction and ex- Warren air. This represents 100% volumetric effi- haust tuning. ciency.W An internal combustion engine has For example, a few readers asked why I [email protected] to operate under a wide range of rpm, load used 15% VE for the idle calculation in the and atmospheric conditions, and therefore last column. That’s a good question. Look at achieves a wide range of VE—generally less the air pressure chart below and note the in- than 100%. verse relationship between engine vacuum A number of factors affect volumetric ef- and intake manifold pressure. The chart in- ficiency. The three notable external factors cludes most of the commonly used air pres- are altitude/atmospheric pressure, tempera- sure scales, but I’ll stick with inches of mer- ture and humidity. The list of internal (en- cury (in./Hg), since this is the most common continued on page 22 Chart: Mark Warren

20 October 2010 Driveability Corner scale for a vacuum gauge and is how slightly less than 50%. So, we take tion caused by a throttle plate in- the weather service generally refer- slightly less than 50% VE and apply it creases efficiency considerably. If you ences atmospheric pressure. to the 30% max possible VE at idle throttled your compressor to 20 Assume we’re at sea level (29.9 and we get a 15% typical VE at idle. in./Hg vacuum or 9.9 in./Hg pressure, in./Hg) and our vehicle is pulling 20 Of course, smaller engines will be it would take three times longer to fill in./Hg vacuum against a closed throt- more efficient at idle and larger en- the tank and therefore require three tle. Looking at the pressure side of the gines will be less efficient. times the electricity. chart, we see that 29.9 in./Hg minus That pesky throttle is a VE killer. A simple but clever technique for the 20 in./Hg vacuum is actually 9.9 improving volumetric efficiency at in./Hg pressure in the intake manifold. light- to medium-load/high-intake- This is the manifold absolute pressure manifold vacuum is to open the ex- (MAP) reading you should be familiar A simple but haust gas recirculation (EGR) valve. with from your scan tool, on a Okay, so you’re driving at 50 mph on MAP/speed density-equipped vehicle. clever technique a level road, throttle opening is low With only 9.9 of the 29.9 in./Hg air and intake manifold vacuum is high. pressure available for the engine to for improving VE To get better fuel economy, start use, the best-case VE here is 9.9 divid- opening the EGR valve. The engine ed by 29.9, or 33%. at light- to will get less oxygen and start to slow Most passenger car engines are de- down, so you roll in more throttle signed for maximum efficiency and medium-load/high- (imperceptible to the driver) to max VE at 2500 rpm; this is where intake-manifold maintain speed. The greater throttle your steady driving load operation usu- opening, combined with the intake ally is. Less than 2500 rpm and more vacuum is to open vacuum drop associated with the than 2500 rpm are compromises. At EGR dumping into the intake, re- idle, the engine’s displacement is way the EGR valve. duces intake vacuum and therefore bigger than necessary to meet the load suction throttling losses. demands, the valve timing is not ideal In 2001, BMW introduced the and the intake and exhaust valves and Valvetronic system, to control intake ports are way bigger than ideal. Diesel engines are not just more effi- valve lift. That system effectively All of this oversizing relative to idle cient due to their higher compression makes the intake valve the throttling airflow leads to low gas velocities and ratios; most diesels are “throttled” device, reducing suction throttling poor combustion chamber turbu- (rpm-controlled) by fuel delivery, not losses and increasing gas velocity at lence. The engine is volumetrically by air intake restriction. Not having the valve. The Valvetronic system re- very inefficient at idle—typically to suck against an induction restric- tains a throttle valve for starting and

Circle #15

22 October 2010 certain vacuum functions; however, intake air velocity is increased, with load operating conditions. Newer the throttle is generally fully open subsequent increases in cylinder tur- electronically controlled turbocharg- during regular engine operation. bulence and better air/fuel mixing. ers control turbo lag and overboost. With an improvement of VE across Also, if cylinder deactivation is ac- The external killer for volumetric the entire operating range, the complished by closing valves, the efficiency is altitude. If you’re going Valvetronic system achieves a 15% in- cylinder compression cycle actually to use VE for engine and MAF diag- crease in engine efficiency and pow- bounces between vacuum and low nostics and you’re at altitude, you’ll er, with a corresponding 15% de- compression. The low compression need to compensate for that altitude. crease in precatalyst emissions. achieved in deactivated cylinders re- Subtract 1 in./Hg from atmospheric When the Cadillac 8-6-4 variable duces top-ring-to-cylinder-wall friction, pressure for every 1000 ft. of altitude. displacement engine came out in the increasing efficiency. Remember, the At 2500 ft., subtracting 2.5 in./Hg 1980s, I wondered where the in- top ring is pushed against the cylinder from 29.9 in./Hg is 27.4 in./Hg. To creased engine efficiency came from. wall with cylinder compression. get a percentage altitude compensa- The engine did not get smaller; it just Want to blow volumetric efficiency tion multiplier at 2500 ft., take the used fewer cylinders under steady- off the charts? Supercharge or tur- 27.4 in./Hg and divide by 29.9 in./Hg state light- to medium-load condi- bocharge the engine. Positive pres- (sea level). This gives you 91.6%. Us- tions. The rotating mass was the sure aspiration yields a lot of benefits. ing this formula, you can see that at same, so no weight was lost. Where Using a smaller engine reduces suc- 10,000 ft., your VE is reduced to was the efficiency gain? With fewer tion throttling losses, because less 66%, compared to at sea level. cylinders activated, the throttle has to time is spent at a low throttle open- Looking for additional information be opened wider for the same load, ing. The smaller engine has less mass on volumetric efficiency? Go to and suction throttling losses are re- and fewer moving parts, reducing in- www.motor.com and check out the duced. Also, with fewer cylinders ertial and friction losses. Gas velocity June and December 2003 and April working and a wider throttle opening, is increased at all engine speeds and 2004 issues.

Circle #16

October 2010 23