Exhaust energy of the diesel

Turbocharged diesel engine exhaust gas from the turbine dynamometer theory can be seen in diesel exhausts, has great energy reserves, accounting for 30% -40% of the fuel energy, and how to use this energy in the exhaust gas turbine, Select the pressurization system is an important basis. Usually two ways to take advantage of exhaust energy, that constant pressure booster system and pulse turbocharging system.

(1) constant pressure booster system

This is characteristic of the supercharging system of each cylinder are connected to the exhaust pipe of an exhaust pipe of the total, as shown in 2. 36 (a) above. Each cylinder of the total exhaust gas are discharged to the exhaust pipe, and then directed to the turbine nozzle ring. In this system, due to the large cross-section and length of the exhaust pipe, and the exhaust gas of each cylinder are alternately added, and the large volume of the exhaust pipe to provide sufficient space for expansion, and therefore the exhaust pipe pressure fluctuations Pr small, into the turbine exhaust pressure remains substantially constant, it is called constant pressure booster systems.

In constant pressure booster systems, exhaust "pulse momentum" no direct impact on the turbine, but the diesel exhaust fumes into the large volume of the exhaust pipe, through the expansion of the exhaust manifold, the friction into heat, The heat has been absorbed by gas, the equivalent of a state under constant pressure and elevated to the higher temperature endothermic e '(Figure 2.35) points. Exhaust manifold into the turbine exhaust is already in a state of constant pressure, and then expanded to f 'points through the turbine. Thus, diesel engine exhaust energy can not be fully utilized in a constant pressure system, the main advantage of the constant pressure can be an area 1-5-ef-1; while virtually no use of "pulse momentum," using only the friction of its transformation energy, that area ee'-f'-fe.

Such as pulse energy use low-pressure turbocharger, large throttling losses. However, when the boost pressure p. When increased, p, correspondingly increased. As can be seen from the figure 2.35, the kinetic energy of the pulse area b-5-eb proportion will decrease, while the constant pressure area will increase the proportion of energy. So high and supercharged diesel engines often requires the use of constant pressure booster systems.

(2) pulse turbocharging system

Characteristics of the system is pressurized pulses of gas cylinder directly and rapidly as possible to the turbine, whereby a constant pressure as much as possible the use of the exhaust system does not use the kinetic energy of the pulse. It is required to be as close to the turbine cylinder, the exhaust pipe is made shorter and thin, and in order to reduce interference between the cylinder pressure wave in the exhaust pipe, the exhaust pipe with a few of the adjacent cylinder combustion exhaust gas spaced [Fig 2. 36 (b)]. Due to the small volume of the exhaust pipe when the exhaust cylinder started, the pressure in the exhaust pipe on the rapidly improving;

And no other in the cylinder while the exhaust gas in the exhaust pipe, the exhaust gas inflow turbine with the exhaust pipe pressure will drop rapidly, the gas pressure inside the cylinder along with the decrease rapidly, and the exhaust of a cylinder, pressure in the exhaust pipe and quickly raised and lowered again. Thus, the formation pressure in the exhaust pipe cyclical fluctuations, the pressure at the turbine inlet also fluctuate, so called pulse turbocharging system.

Pulse pressurization system to obtain good exhaust pulse, and quickly reduce the pressure of exhaust gas after opening the exhaust valve early to increase the scavenging effect, each time the exhaust gas cylinder connected to the exhaust pipe should be staggered , and do not re-board (or overlap very little). For example, four-stroke diesel engine exhaust sustained angle of about 240 °, and therefore each exhaust pipe connected cylinders should not exceed: 720 ° / 240 ° = 3个. The same group of fewer than three cylinders of course does not interfere exhaust, but not a continuous supply of gas, the exhaust gas energy transfer process can be used to increase the loss of the turbine intake intermittent losses caused by increased intake section, the turbine efficiency is reduced, the number of cylinders in the same group with the best 3. For example, a six-cylinder four-stroke diesel engine firing order is 1-5-3-6-2-4, each cylinder firing interval angle of 720 ° / 6 = 120 °, grouped according to the above principles to get a cylinder, 2-cylinder, 3-cylinder as a group, 4-cylinder, 5-cylinder, 6-cylinder to another group.

In summary, the pulse relative to the constant pressure booster system booster exhaust energy utilization system has obvious advantages. However, with the increase of the supercharging pressure, the pulse energy of the exhaust gas turbocharger in decreased utilization of the advantages, the exhaust pipe volume pressurized pulses is small, when the engine load is changed, the exhaust gas pressure wave can be changed immediately. and passed quickly to the turbine, thus better acceleration performance; But instantaneous flow exhaust pulse turbocharging system is cyclical changes, the instantaneous maximum flow t flow pressure system over a given (equivalent to the average flow pulse pressurization system) large . Therefore, the larger the size of the pulse of the turbine, the exhaust pipe structure is also more complicated and restricted the number of each cylinder is connected to the exhaust pipe. Therefore, when a low pressurization, the pressurization system is advantageous pulse, and in supercharging, the system pressure should be used.