Combustion system design method based on target heat release rate
Disclosed is a combustion system design method based on a target heat release rate, which belongs to the technical field of diesel engine combustion chamber design. The method includes: obtaining an ideal heat release rate based on Sabathe-Miller cycle; simulating the ideal heat release rate based on a double-Wiebe function and obtaining the target heat release rate; constructing a mapping relation among the heat release rate, piston geometric parameters and fuel injection parameters, which includes target start of combustion being an function of fuel injection timing and ignition delay, premixed combustion parameters being functions of throat radius, injection pressure and nozzle diameter, and diffusion combustion being a function of piston pit depth; solving target piston geometric parameters and target fuel injection parameters based on the mapping relation; and then designing a combustion system. The method does not depend on experience and multi-scheme design, greatly shortens the combustion system design.
1. A combustion system design method based on a target heat release rate, comprising following steps:
step S 1 : obtaining an optimal and ideal heat release rate based on a Miller and Sabathe cycle coupled model;
step S 2 : simulating the optimal and ideal heat release rate based on a double-Wiebe function and obtaining the target heat release rate;
step S 3 : constructing a mapping relation among the target heat release rate, piston geometric parameters and oil injection parameters, wherein, the mapping relation comprises a target combustion starting point being an oil injection timing and ignition delay period function, a premixed combustion duration being a combustion chamber throat radius, oil injection pressure and injection orifice diameter function, a premixed combustion mass being an oil injection pressure and injection orifice diameter function, and diffusion combustion being a piston pit depth function;
step S 4 : solving target piston geometric parameters and target oil injection parameters based on the mapping relation and the target heat release rate; and
step S 5 : designing a combustion system according to the target piston geometric parameters and the target oil injection parameters.
2. The combustion system design method based on the target heat release rate according to claim 1 , wherein the step S 1 specifically comprises:
step S 101 : constructing an effective thermal efficiency calculating zero-dimensional model based on a Miller and Sabathe coupled cycle; and
step S 102 : analyzing an influence rule of an isochoric degree and a Miller degree on effective thermal efficiency based on the effective thermal efficiency calculating zero-dimensional model, and obtaining the optimal and ideal heat release rate.
3. The combustion system design method based on the target heat release rate according to claim 2 , wherein the isochoric degree is a ratio of an isochoric combustion fuel mass to a total fuel mass, and the Miller degree is an IVC time volume ratio of a conventional cycle to a Miller cycle.
4. The combustion system design method based on the target heat release rate according to claim 1 , wherein the step S 2 specifically comprises:
step S 201 : simulating the ideal heat release rate by the double-Wiebe function based on a one-dimensional GT simulation model and obtaining Wiebe function characteristic parameters;
step S 202 : optimizing the Wiebe function characteristic parameters in combination with preset diesel engine limiting conditions; and
step S 203 : obtaining the target heat release rate according to the optimized Wiebe function characteristic parameters.
5. The combustion system design method based on the target heat release rate according to claim 4 , wherein the Wiebe function is:
dx
1
d
φ
=
[
(
m
p
+
1
)
*
6.908
*
(
1
2
τ
)
m
p
+
1
(
φ
-
φ
B
)
m
p
e
-
6.908
(
1
2
τ
)
m
p
+
1
(
φ
-
φ
B
)
m
p
+
1
]
(
1
-
Q
d
)
,
x l is a heat release volume, m p is a constant, τ is ignition delay, φ is a crank angle, φ B is a crank angle corresponding to a combustion starting point, and Q d is a combusted fuel mass fraction.
6. The combustion system design method based on the target heat release rate according to claim 1 , wherein the step S 3 specifically comprises:
step S 301 : dividing a heat release rate into a premixed stage and a diffuse stage with a spray wall-impact time as a dividing point, and respectively studying relations among characteristic parameters in the premixed stage, oil injection parameters and combustion chamber shape parameters and among characteristic parameters in the diffuse stage, the oil injection parameters and the combustion chamber shape parameters; and
step S 302 : respectively determining a target combustion starting point being an oil injection timing and ignition delay period function, a premixed combustion duration being a combustion chamber throat radius, oil injection pressure and injection orifice diameter function, and a premixed combustion mass being an oil injection pressure and injection orifice diameter function in the premixed stage, and determining diffusion combustion being a piston pit depth function in the diffuse stage.
7. The combustion system design method based on the target heat release rate according to claim 6 , wherein the target combustion starting point is the oil injection timing and ignition delay period function:
θ SOC =θ SOI +θ id
θ id =C 1 exp( C 2 /T a ) d 0 C 3 ρ A C 4 P inj C 5 O 2 C 6 ,
θ SOC is a combustion starting point, θ SOI is an oil injection starting point, θ id is an ignition delay period, C 1 -C 6 are constants, T a is an environmental temperature, ρ A is an environmental density, d 0 is an injection orifice diameter, P inj is an injection pressure, and O 2 is an oxygen concentration.
8. The combustion system design method based on the target heat release rate according to claim 6 , wherein when a combustion chamber throat is a wall-impact position, the premixed combustion duration is the combustion chamber throat radius, oil injection pressure and injection orifice diameter function:
√{square root over (( r th ) 2 +( x (φ)+ I cl ) 2 )}= K p (( P i −P a )/ρ A ) 0.25 d 0 0.5 t 0.5 ,
r th is a combustion chamber throat radius, x(φ) is a piston displacement at a crank corner, l cl is a clearance height, K p is a constant, P i is an injection pressure, P a is a backpressure, ρ A is an environmental density, d 0 is an injection orifice diameter, and t is a duration from an oil injection starting point to a wall-impact time, namely the sum of the premixed combustion duration and an ignition delay period.
9. The combustion system design method based on the target heat release rate according to claim 6 , wherein the premixed combustion mass is the oil injection pressure and injection orifice diameter function:
m
net
=
k
ρ
d
0
2
(
P
i
-
P
a
)
0.5
·
(
0.5
a
·
ln
(
P
i
-
P
a
K
p
ρ
A
d
0
)
-
b
)
,
m net is a premixed combustion mass, k ρ is a model parameter, d 0 is an injection orifice diameter, P i is an injection pressure, P a is a backpressure, a, b and K p are constants, and ρ A is an environmental density.
10. The combustion system design method based on the target heat release rate according to claim 6 , wherein the diffusion combustion is the piston pit depth function:
HRR
∝
m
(
0.5
k
f
w
2
V
2
3
+
0.5
k
f
w
1
V
1
3
)
,
HRR is a diffusion combustion heat release rate, m is a diffusion combustion fuel mass, k f w1 and k f w2 are turbulence intensities corresponding to discrete volumes respectively, and V 1 and V 2 are discrete volumes respectively.